U. S. Dept. Agriculture Handbook No. 18
Soil Survey Manual
SOIL SURVEY STAFF
Bureau of Plant Industry, Soils, and
This is a Revision and Enlargement of U. S. Department
of Agriculture Miscellaneous Publication 274, the
Soil Survey Manual, Issued September 1937,
and Supersedes it.
Effective 1952, the Soil Survey was transferred from the former
Bureau of Plant Industry, Soils, and Agricultural Engineering to the
SOIL CONSERVATION SERVICE
Reissued in October 1962 with no change in text.
Agricultural Research Administration
UNITED STATES DEPARTMENT OF AGRICULTURE
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, I).(. 2011)2 Price $.1.75 (Buckram)
Issued August 1951
Introduction ........................ ............... V
Soil and landscape .................................... 1
Character of soil maps and reports. ...................... 11
Purpose of soil maps and reports. ....................... 23
Preparation for field work. .............................. 43
Plotting and assembly of field data ...................... 71
Examination and description of soils in the field ........... 123
Parent materials of soils ............................... 147
Land form, relief, and drainage ......................... 155
Identification and nomenclature of soil horizons ........... 173
Soil color ................... ..........................189
Soil texture, coarse fragments, stoniness, and rockiness. 205
Soil structure ........................................ 225
Soil consistence ....................................... 231
Soil reaction ......................................... 235
Special formations in soil profiles ....................... 239
Organic matter and roots .............................. 245
Accelerated soil erosion ................................ 251
Vegetation .......................................... 271
Land use ........................................... 275
Units of soil classification and mapping .................. 277
The soil mapping legend ............................... 313
Plotting soil boundaries in the field. ...................... 321
Collection and examination of soil samples................ 327
Estimation and mapping of salts and alkali in the soil ...... 339
Yield predictions and soil management practices.......... 365
Soil correlation and inspection .......................... 397
Soil grouping on the map................................ 403
The soil survey report.................................. 409
Reconnaissance soil mapping ....................... .. .435
General bibliography ....................... .......... I
Special bibliography of soil surveys ....................... 447
Appendix I. Map preparation with the plane table ........ 455
Appendix II. Map preparation with compass traverse ...... 461
Appendix III. Notes on map compilation and reproduction. 465
Appendix IV. Sample descriptions of soil series........... 471
Appendix V. Guide to map scales ........................ 487
Index .................. ...... ..................... 489
The Soil Siurvcy Manual is intended for use by soil scientists
engaged in soil classification and mapping. Attention is directed
primarily to problems and methods of making and interpreting
detailed basic soil surveys in the United States and territories.
The earlier edition,' published in the autumn of 1937, reflected
the developments growing out of the ideas, work, and publications
of hundreds of scientists since the beginning of the United States
Soil Survey in 1899. Substantial progress has been made since
1937 in the soil survey itself and in related fields of soil research.
Further, soil surveys are now used by more people, in more ways,
and, above all, with more precision than formerly.
The increased use of soil maps and interpretations has led to
increased testing of the results, both scientifically and practically.
Inadequacies appeared that required correction. Continually, new
knowledge about soils needs to be incorporated into the classifica-
tion and into the interpretations. New research methods and new
cartographic methods need to be evaluated, adapted, and used as
they are appropriate to improve soil surveys and to reduce their
Nearly the whole of the earlier edition of the Manual has had
to be revised. Although some appear to be drastic, few of the
revisions are out-and-out changes; most of them are modifications
and elaborations to achieve the specificity and completeness
required to make the final results more nearly quantitative and
more useful. For example, essentially all soil mapping is now done
on aerial photographs, and the discussion of the older carto-
graphic methods has been condensed in appendices.
Some new terms have been added and a great many redefined,
especially to permit increased accuracy. This process of redefining
will need to go on as long as soil research continues. The discovery
of new relationships and the formulation of new concepts require
an expansion of language.
Many of the technical terms used in soil science are common
words, taken out of the body of language and given precise and
sometimes unusual meanings. A large part of them originated as
folk terms among rural people. Such words as "loam," "texture,"
"structure," "heavy," "light," "profile," "horizon," and even "soil"
may have a deceptive familiarity to the layman using the language
of soil science. Similar technical words have arisen in the same
way in the other languages, often with slightly different shades of
meaning, not revealed in the ordinary lexicon. The meaning of
coined words, like "Lithosol" and "illuviation," or of those taken
bodily from other languages, like "gley" or "Chernozem," once
SSOIL SURVEY MANUAL. U.S. Dept. Agr. Misc. Pub. 274, 136 pp., illus. 1937.
U. S. DEPT. AGRICULTURE HANDBOOK 18
learned, are not so likely to be confused with other meanings as
are redefined common words.
Even though newly coined words are more easily defined than the
older ones are redefined, their use on soil maps and in soil survey
reports intended for the general reader is limited. For some new
concepts a writer has no alternative to technical terms. These
he needs to define for the general reader. Commonly, however, the
older more general words must be used in soil survey reports,
insofar as possible, in order to capitalize on the readers' present
understanding. But in the scientific work itself specific terms
should be used in the sense of accurate definitions. Thus there is
no escape from a certain amount of "double language."
Need of accurate definition.-Special effort has been made in this
revised text of the Manu al to define terms and to use them as
specifically as possible. Since the early edition, much progress has
been made toward uniformity of terminology among soil scientists.
Better definitions are still needed within our own language, and
especially better transliterations among the various languages.
Some nearly arbitrary selection among alternatives has been
necessary in the Manual.
A separate glossary is not included because much duplication
would result and because many definitions are clearer when set
within an explanatory context. Where definitions might lead to
long and highly technical statements, explanations are given
instead. Page numbers of the Index in bold type refer to definitions
and explanations of the terms.
The relationships of the soil survey to other researches have
deepened and broadened as its uses and interpretations have
expanded. It has seemed that this Manual should be broad enough
in scope to lead into the most important of these relationships,
but it cannot develop them in detail. Even the field of soil classi-
fication, above the lower categories, lies mostly outside of its scope.
A few references to fuller discussions are given in the text, and
a suggested reference shelf is included near the end.
Since the earlier edition was prepared (during 1935-36) all
phases of the work have been under study by the Soil Survey staff.
Following intensive study and revision, mimeographed copies of
new statements about many individual subjects treated in the
Manual were circulated both for guidance in making soil surveys
and for criticisms and suggestions. Since all the basic soil survey
work in the United States is carried on cooperatively with the
State land-grant colleges and universities, several scientists in
those institutions have helped a great deal in criticizing state-
ments on special subjects and the draft of this edition of the
Mantal. Besides, informal cooperation is carried on with the
research organizations of several foreign countries. Scientists
from these countries have given us the benefit of their valuable
experiences and judgments. Several read all or parts of the draft
manuscript and made valuable suggestions for its improvement.
Other modifications can be expected, especially in classification
and nomenclature, as our knowledge and experience advance.
SOIL SURVEY MANUAL
Some prospective changes are under study and are mentioned
here and there in the text.
The authors have had great help from the criticisms and com-
ments given by readers of the lManual published in 1937. It is
hoped that readers of this revised edition will note errors and
omissions and call them to the attention of the Soil Survey staff
so that any subsequent edition may be improved.
Arrangement of topics.-It is assumed that most readers of
the M3noial will have had training equivalent to that of a graduate
holding the Bachelor of Science degree from a curriculum in soil
science like that ollicially recommended by the Soil Science Society
of America.: It is expected that many readers will need to carry
on collateral reading in soil classification, general soil science,
geology, interpretation of aerial photography, geography, eco-
nomics, and general agriculture.
The authors have further assumed that soil survey party chiefs
and those wishing to prepare for such responsibilities will want
to study all parts of the Maliol. T-,. > '..,., the topics are
arranged roughly in the order that problems arise in starting,
carrying out, and completing a soil survey, although, of course,
a party chief must have a view of all aspects to begin with, since
the several phases of the work are closely interrelated. It is
assumed that others who are not concerned in the whole job may
find the Moanal a helpful reference for particular items that can
be located from the table of Contents or the Index.
Soil Sci. Soc. Amer. Proc. 6: 507. 1941.
SOIL AND LANDSCAPE
First let us briefly review the working concepts of soil and of
the principles of scientific method upon which this Manual is
based. These have been formulated only after many years of
trial and error.
When the Soil Survey began in the United States, more than
50 years ago, there was no organized body of knowledge that we
have come to know as soil science. This is not to say that nothing
was known about soils. Indeed farmers had learned a great deal
through experience over the centuries, and much of their knowl-
edge had been brought together in several compilations, some as
early as Roman times. With the rise of agricultural chemistry
during the nineteenth century, more was learned about soils that
was useful. Yet it was not until some time near the end of the
century that the knowledge about soils gained from farming,
from agricultural chemistry, from biology, and from geology was
coordinated. Nor could it be coordinated without some unifying
concept of the soil itself.
The early concepts.-With few exceptions, like Hilgard's ideas,'
the notions of soils held by soil workers at the time the Soil Survey
began were based upon assumptions stemming mainly from the
ideas of the great German chemist, Liebig, as modified and per-
fected by agricultural chemists and plant physiologists working
on samples of soil in laboratories and greenhouses and on small
plots of soils in the field. The soils were rarely examined below
the layer turned in regular tillage. The assumption of soil
character, or working theory, which was more or less uncon-
sciously conceived, may be briefly summarized as the balance-sheet
theory of plant nutrition or the-soil-is-like-a-bank idea. Soils were
considered to be more or less static storage bins for plant
nutrients that could be used by plants but had to be replenished
as used. Of course, the amounts of nutrients removed from soil
by harvested crops and those returned in manure, lime, and
fertilizers are important to an understanding of soil productivity;
'The soil scientists of today cannot help being amazed at the general
neglect of E. W. Hilgard's important and pioneer work, first in Mississippi
(GEOLOGY AND AGRICULTURE OF TIIE STATE OF MIISSISSIPPI. 3,91 pp., Jackson,
Miss. 1860.); then in the Cotton Belt as a whole (A REPORT ON COTTON PRO-
DUCTION IN TIE UNITED STATES; ALSO EMBRACING AGRICULTURAL AND PIIYSIO-
GEOGRAPHICAL DESCRIPTIONS 01 TIE SEVERAL COTTON STATES AND CALIFORNIA
in volumes 5 and 6 of the 10th Census of the United States. Washington.
1884); and finally in California (soILS; THEIR FORMATION, PROPERTIES, COM-
POSITION, AND RELATIONS TO CLIMATE AND PLANT GROWTH IN THE HUMID
AND ARID REGIONS. 593 pp., illus. New York and London. 1906.).
U. S. DEPT. AGRICULTURE HANDBOOK 18
but a great deal more is needed for our understanding of soils
and their management requirements. In fact, this simple balance-
sheet theory, by itself, has but little prediction value.2
The early geologists generally accepted this notion of soil
fertility. They filled the conceptual storage bin with ground rock
of various sorts-granite, sandstone, calcareous till, and the like.
They went further, however, and showed how the weathering
processes modified this material and how the geological processes
of landscape formation used it in the construction of land forms,
such as glacial moraines, alluvial plains, loessial blankets, and
marine terraces. Shaler's monograph on the origin and nature of
soils: went about as far as it was possible to go with this geological
concept of soils; although many details were added in Merrill's
Professor Milton Whitney and his coworkers in the new soil
research unit of the United States Department of Agriculture,
established near the end of the nineteenth century, were impressed
by the great variations among natural soils-persistent variations
in no way due to the effects of agricultural use. Special emphasis
was given to soil texture and to the ability of the soil to furnish
plants with moisture as well as nutrients. Professor F. H. King
of the University of Wisconsin was also emphasizing the physical
characteristics of soils."
The Soil Survey began in response to the recognized need for
helping farmers locate themselves on soils responsive to manage-
ment and, once located, for helping them to decide what crops
and what management practices were best for the particular
kinds of soil on their farms.
In early surveys, soils were conceived to be the weathering
products of recognized geological formations, defined by land
form and lithological composition. Many of the earlier field
workers were trained in geology, because only geologists were
skilled in field methods and in the scientific method of correlation
appropriate to the field study of soils.
Shortly after field work began, it became obvious that many
important soil characteristics were not definitely related to either
broad land form or rock type. It was noted that naturally poorly
drained soils have different characteristics than naturally well
drained soils, and that sloping soils are unlike level ones. On
broadly similar glacial till from Maine to Montana, and down
to the Ohio River, markedly contrasting soils are developed in
SSee KELLOGG, CHARLES E. CONFLICTING DOCTRINES ABOUT SOILS. Sci.
Monthly 66: 475-487. 1948.
SEALER, N. S. TIE ORIGIN AND NATURE OF SOILS. U. S. Geol. Survey
Ann. Rpt. 12: 213-345, illus. 1891.
MERRILL, G. P. A TREATISE OF ROCKS, ROCK-WEATIERING AND SOILS. New
ed., 411 pp., illus. New York and London. 1906.
See for example, KING, F. H. A TEXTBOOK OF THE PHYSICS OF AGRICUL-
TURE. Ed. 3, 604 pp., illus, Madison, Wis. 1910.
SOIL SURVEY MANUAL
the various climatic and biotic zones. Yet for several years the
geological view dominated in the field, and the balance-sheet
theory of plant nutrition in the laboratory. Although they were
taught in many clarv-:rooms until the late 1920's, neither theory
actually worked well in the field as a basis for reliable predic-
tions to farmers. As a consequence, all sorts of special little
concepts were formed that broke down in contradiction when
applied to a great continental area like the United States.
Broader and more useful concepts of soil were developing
among some American soil scientists, especially Hiigard. The
necessary data for formulating these broader concepts came in
rapidly from the field work of the Soil Survey during the first
decade of its operations. After Hilgard, the longest reach toward
a more satisfactory concept was made by Coffey."
Soil profiles and the concept of individual soils.-Meanwhile,
beginning in 1870, a new concept of soil was developing in the
Russian school of soil science.7 The results of this work became
generally available to Americans through the publication of
Glinka's great textbook in German and especially through its
translation into English by C. F. Marbut.' Boiled down to its
essentials, soils in the Russian concept were conceived to be
independent natural bodies, each with a unique morphology and
resulting from a unique combination of climate, living matter,
parent rock materials, relief, and time. The morphology of each
soil, as expressed in its profile, reflected the combined effects of
the particular set of genetic factors responsible for its develop-
This was a revolutionary concept, as important to soil science
as anatomy to medicine. The soil scientist did not need to depend
wholly upon inferences from the geological nature of the rocks,
or from climate, or from other environmental factors, considered
singly or collectively; rather, he could go directly to the soil itself
and see the integrated expression of all these in its morphology.
This concept made it not only possible but necessary to consider
all soil characteristics collectively, in terms of a complete, inte-
grated natural body, rather than individually. In short it made
a soil science possible.
CoFF'Y, (. N. A STUDY OF THE SOILS OF THE UNITED STATES. U. S. Dept.
Agr. Bur. Soils Bul. 85, 11.1 pp., illus. 1912.
SSee the following references:
GElImOIZ, K. K. SOIL-ABSO!RBINGI COMPLEX AND THE ABSORBED SOIL CATIONS
AS A BASIS OF GENETIC SOIL CLASSIFICATION. (Trans. by S. A. Waksman)
Nossov Agr. Expt. Sta. Iaper 38, 29 pp. Lenigrad. [In papers on soil reaction
KELLOGG. CHARLES E. RUSSIAN CONTRIBUTIONS TO SOIL SCIENCE. Land
Policy Rev. 9: 9-14. 1940.
NEUST'IUEv, S. S. GENESIS OF SOIL. Russ. Pedol. Invest. 3, Acad. Sci.,
98 1pp. Leningrad. 1927.
GLINKA, K. 1). THE (;IEAT SOIL GROUPS OF THE WORLD AND THEIR DEVELOP-
MENT. (Trans. from the German by C. F. Marbut.) 235 pp. Ann Arbor,
4 U. S. DEPT. AGRICULTURE HANDBOOK 18
With the early enthusiasm for the new concept and for the
rising new discipline it made possible-soil science"-some went
so far as to suggest that the other sciences were unnecessary to
soil study. Perhaps some extreme statements in this tone were
made to declare a certain sense of autonomy and freedom from
the older concepts of geology and agricultural chemistry rather
than from thoughtful conviction. Certainly the reverse of inde-
pendence from other sciences was true, for besides laying the
foundation for a new science with its own principles, the new
concept made the other sciences even more useful. In soil mor-
phology, the soil scientist found a firm basis on which to classify
the results of observation, of experiments, and of practical experi-
ence, and to develop principles of prediction value.
Under the intellectual leadership of C. F. Marbuto0 the new
concept was further broadened and adapted. As first explained,
this concept emphasized individual soil profiles-soils at points on
the earth's surface-even to the subordination of external soil
features and surface geology. This weakness become more clearly
evident in the United States, perhaps, because of the great
emphasis upon detailed soil maps for their practical prediction
value. Progress was rapid because of the large body of important
field data already accumulated. By 1925 a large amount of mor-
phological and chemical work was being done on soil profiles
throughout the country. The data available around 1930 were
summarized and interpreted in accordance with this concept, as
STerminology is still confused. A large amount of applied soil science, and
even some fundamental soil science, is still included under agronomy in sev-
eral colleges and universities in the United States. Partly to differentiate
it from applied agricultural science, another large field of application is
termed "soils engineering." Terms like "soils geology" and "forest oilis'
are also used for parts of the field of soil science. "Soil t..!ii0. i.,. has
been used in the narrow sense of soil manipulation-drainage, irrigation,
erosion control, tillage, and the like-and also in the broader sense of all
applied soil science. Similarly, "soil conservation" is commonly used not only
in the narrow sense of erosion control but also in various broader senses up
to "soil management for sustained production."
In Europe generally, the word "scientist" has a somewhat more exalted
connotation than in the United States. Thus individuals hesitate to call them-
selves "soil scientists." They prefer a single word like "pedologist." Un-
fortunately, in the United States, pedology has come to mean only those
phases of the more general field of soil science that relate directly to soil
morphology, genesis, and classification. In this sense pedology is even too
narrow for the work of the Soil Survey. Further, the term "soil science"
has at least some self-evident connotation to the layman. The authors see no
better alternative in the United States than "soil science" for the general
field-for the science that treats of soils, including their nature, properties,
formation, functioning, behavior, and response to use and management. In
many countries this also defines "pedology" as the term is now used in them.
"' See the following:
MARBUT, C. F. THE CONTRIBUTION OF SOIL SURVEYS TO SOIL SCIENCE. SoC.
Prom. Agr. Sci. Proc. (1920) 41: 116-142, illus. 1921.
--- A SCIIEMIE FOR SOil CLA\SSFI('ICATION. 1st Inlernail. Cong. Soil Sci.
Comm. 5, Proc. anld apers 4: 1 31, illus. 1928.
SoIL S'IENCEI SoCITY ,or AiMERICA. LIFEi AND \WORK OF C. F. MA VRHUT. 271
pp., illus. Columbia, Mo. 1942.
SOIL SURVEY MANUAL 5
viewed by Marbut, in his great work on the soils of the
Marbut always emphasized strongly that soil classification
should be based on soil morphology, since theories of soil genesis
were both ephemeral and dynamic. He was led to emphasize this
point so much-perhaps even to overemphasize it-because of the
previous errors made by acceptance of the balance-sheet theory
and the geological concept under which soils had been assumed
to have certain characteristics without the scientists taking the
trouble to examine the soils to see whether they were like they
had been assumed to be. Marbut was trying to make the point
abundantly clear that examinations of the actual soils were
essential for developing a system of soil classification and for
making soil maps of prediction value. (This still needs emphasis
today. Even yet schemes of soil classification and mapping are
occasionally put forward that are designed to avoid the work of
Extreme interpretations of Marbut's emphasis upon morphology
as the basis for classification led to the suggestion that the soil
classifier could neglect genetic principles and relationships. Such
extremes should be avoided. A soil is not really understood until
its genesis and the reasons why it varies from other soils are
known. Not until the morphology and genesis of a soil are known
can research to discover new and improved management systems
be planned most effectively. Without such organized knowledge,
purely empirical mass plot work alone must be resorted to with
the hope that something will work. This is the situation now with
many tropical soils. The Ground-Water Laterite soils are an
example. Until their genesis is worked out, finding practical
systems of soil management by empirical plot trials alone seems
nearly hopeless. Fundamental soil research should be emphasized
more as a basis for classification, applied research, and the
invention of new techniques.
One may conceive, perhaps, of the development of an accurate
system of soil classification on the basis of morphology alone;
but in practice it is doubtful that completely satisfactory results
can be had. Besides accurate morphology, genesis is needed to
guide the work and to test the results. Neither one nor the other
can be neglected. Yet in the meantime, classification of soils of
obscure genesis shall need to be handled as well as possible,
largely on the basis of morphology alone.
Soils as dynamic three-dimensional landscapes.-The concept of
soil was gradually further broadened and extended around 1930
and the years immediately following.' This revision in concept
was not so dramatic as the earlier one; it was more a matter of
consolidation and balance. Previously the major emphasis had
been on the soil profile. Soil profiles come very near to occupying
MARBUT, C. F. SOILS OF TITE UNITED STATES. Ih U. S. Dept. Agr. Atlas
of American Agriculture, pt. :, Advance Sheets No. 8, 98 pp., illus. 1935.
See KELLOGG, CHARLES E. MODERN SOIL SCIENCE. Amer. Scientist 36.
517-536, illus. 1948.
6 U. S. DEPT. AGRICULTURE HANDBOOK 18
single points on the earth's surface; whereas soils have shape
and area, breadth and width, as well as depth. Morphological
studies began to be extended from single pits to long trenches
or to a series of pits over a soil area. The morphology of a soil
is expressed by a range of profiles from a modal profile, not by a
single profile or even by a typical one. Further, early emphasis
upon genetic soil profiles had been so great as to suggest that in
the absence of such genetic profiles, as in a young Alluvial soil,
there was no "true" soil! A sharp distinction had been drawn
between rock weathering and soil formation. Although distinction
between these sets of processes is necessary, it is equally necessary
to recognize that rock weathering and soil formation are sets of
processes going on at one time in the same landscape. Soils are
dynamic not only as soil profiles but also as landscapes.
Clarification and broadening of the concept of soil also grew
out of the continuing emphasis upon detailed soil mapping and
especially with the emphasis upon predictions of estimated yields
for adapted crops under physically defined sets of management
practices for each kind of soil shown on the maps. Many of the
older descriptions of soils had not been sufficiently quantitative,
and the classificational units had been too heterogeneous for
making the yield predictions and management predictions needed
for individual farm planning. The use of air photos, begun during
the late 1920's, had greatly increased the accuracy of plotting soil
boundaries. To meet the needs for farm planning, greater precision
of interpretation was also required. This development of schemes
for summarizing predicted yields and soil behavior under defined
sets of management practices not only made the soil survey far
more useful but also forced a reconsideration of the very concept
of the soil itself.
Soil defined.-First of all, soil is the natural medium for the
growth of land plants, whether or not it has "developed" soil
horizons. Soil in this sense covers land as a continuum, except
on rocky slopes, in regions of continuous cold, in very salty playas,
and elsewhere that the cover of soil disappears. Soil has many
forms. Its characteristics in any one place result from the
combined influence of climate and living matter, acting upon the
parent rock material, as conditioned by relief, over periods of
time, including the effects of the cultural environment and man's
use of the soil.
In studying the characteristics of soil and in predicting its
potentialities for use, we cannot work with the whole continuum
at once. Individual kinds of soil must be recognized. To make use
of experience and of the results of research, classification becomes
a necessity. It is through classification, as a tool, that we organize
our knowledge and remember it, see relationships among soils
and between them and their environment, and formulate principles
of prediction value.
In the sense of an individual in the continuum, a soil is a
dynamic three-dimensional piece of landscape that supports plants.
It has a unique combination of both internal and external charac-
SOIL SURVEY MANUAL 7
teristics that have definable ranges of expression. Each individual
kind of soil has a modal set of characteristics within the limits
set by our logic. Its upper surface is the surface of the land; its
lower surface is defined by the lower limits of soil-forming
processes; and its sides are boundaries with other kinds of soil,
where changes occur in one or more differentiating characteristics,
related, in turn, to one or more of the genetic factors. Through
research, the behavior of soils under defined conditions can
Many thousands of unique kinds of soil exist in the world-
as many as there are significant combinations of the genetic
factors. The characteristics of each can be learned through
observation and research in the field and in the laboratory. The
history of a soil and its potentialities are contained in these
characteristics, considered collectively. The ;,fl,,i, ,/ on soil
behavior of (On one characteristic, or of a variation in any one,
depends uvpon the others in the comrbinttion. (Probably more
faulty predictions about soils result from failures to recognize
this principle than from any other error.) A general system of
soil classification comprehends all observable relevant charac-
Soils, then, are landscapes as well as profiles. The soil mapper
has always recognized this in drawing soil boundaries. Commonly
they come at the foot of an escarpment, at the margin of the
swamp forest, or at some other obvious boundary among natural
landscapes. The hardest soil boundaries of all to plot are those
that can be located only through repeated examination of soil
profiles because the controlling genetic variable is obscure. In
detailed soil mapping, examinations of soil profiles are always
essential to test the location of boundaries and to identify the
In the concept of soil as landscape, slope is an important soil
characteristic. Soils, like other natural bodies, have shape.
Formerly one wrote "soils on sloping land;" now we say simply,
and more correctly, "sloping soils." Temperature is an important
soil characteristic, even though it cannot be preserved in samples.
The same may be said of stoniness and microrelief. A soil is a
natural thing out-of-doors. Like a river or a glacier or a volcano,
it cannot be brought into the laboratory. Thus, no matter how
much and how valuable are the data we obtain on soil samples
in the laboratory, the final synthesis into predictions can be made
accurately only on the basis of all the characteristics of a soil
as a landscape out-of-doors.
Since one cannot distinguish accurately under all conditions
between "soil" and "not-soil," a precise general definition is im-
possible. The same is true of other well-understood basic words
like "house," "plant," or "stone." Many thousands of individual
kinds of soil have been defined. In most of these, but not all, one
can decide clearly between the soil and the not-soil beneath it.
Ordinarily we think of soil as including the upper part of the
earth's crust that has properties different from the rock material
U. S. DEPT. AGRICULTURE HANDBOOK 18
because of the influence of the soil-forming factors. Yet the
definitions of many individual soils must go further and include
layers beneath that influence their behavior. Then, some soil-
landscapes that support plants gradually thin to moss-covered
rock and finally to bare rock with no clear separation between
soil and not-soil that applies generally. Plants may be grown
under glass in pots filled with samples of soil, with peat, with
sand, or even with water. Under proper conditions all these media
are productive of plants but some are not-soil. Plants even grow
on trees; but trees are regarded as not-soil. Yet perhaps the most
important quality of soil is its productivity for plants.
The following general definition of soil may serve those who
need one: Soil is the collection of natural bodies occupying por-
tions of the earth's surface that support plants and that have
properties due to the .: irated effect of climate and living
matter, acting upon parent 'material, as conditioned by relief,
over periods of time.
Scientific methods.-To understand the significance of any par-
ticular soil characteristic, or of any one genetic factor, sets of
soil characteristics must be defined and compared. These sets are
the units in soil classification.': To find the place of an unknown
soil in the system of classification, or to understand the relation-
ship of one soil to others in the system, the sets of characteristics
are compared. This method of scientific correlation is the principal
tool in soil classification.
Because of its universe and methods, soil science does not fit
neatly with the physical sciences, the biological sciences, or the
earth sciences. It is all three, but is not any one exclusively.
Principles and methods from all three are used, in addition to
those that are peculiar to soil science itself.
A large and growing body of fundamental scientific knowledge
is the concern of soil science and of no other discipline. These
facts emphasize the importance of seeing the science as a whole.
No matter how much a soil scientist specializes, he must maintain
a broad view of the whole field. In some sciences, like chemistry
and plant physiology, for example, dependence is placed chiefly
on one general scientific method-the experimental method. Un-
consciously, some have assumed that the experimental method is
the only method in science. Certainly it is a very useful one in
soil science. With this method, the specific effects of variations in
individual soil characteristics, and groups of soil characteristics,
can be observed under defined conditions. The scientist then sets
up experiments on plots representing an individual soil in which
he can control the other variables, or at least account for their
effects, besides the one under study. A large part of what has
been learned about the behavior of specific kinds of soil has
resulted from controlled field experiments, natural experiments,
and the analyses of the records of operating units-farms, gardens,
Sec section on Units of Soil Classification and Mapping.
SOIL SURVEY MANUAL
Yet a great many matters must come under scientific study
that cannot be subjected to experiment. For example, we can get
at the relative influences of different climatic regimes on the
genesis of unlike soils from granite, say, through the use of both
the experimental method and the method of correlation. The
experimental method deals with soils at small places, almost
points. Through the method of correlation the sets of data from
different places are compared and principles developed from
them that fit the facts.
Useful results can only come out of those experimental plots that
are fair samples of a defined kind of soil. To interpret the results,
either for an understanding of soils or for predictions about their
behavior, they must be synthesized in terms of defined soil units.
This is the function of soil classification. Its stuff comes from
observation and the experimental method; its working tool is the
method of logical scientific correlation.
Soil classification depends upon the results from all branches
of fundamental and applied soil science. On the other hand, the
results from the other branches of soil science can only be
synthesized for accurate application through soil classification,
whether soil maps are made or not. Soil classification has been so
intimately associated with soil mapping, for which it is an imme-
diate necessity, that some individuals in other branches of soil
science have not always seen that, in the long run, soil classifica-
tion is just as important to their work, especially to the orderly
application of their results.
In applying soil science to forestry, farming, grazing, and
engineering, some means must be had for recognizing the indi-
vidual units of the classification system in the field. Few people
among those needing to use the principles and predictions of soil
science can identify these units. Thus it is essential to have soil
maps. Assuming an adequate system of soil classification, with
the units consistently named and with reliable predictions, an
accurate soil map makes possible the orderly application of
our knowledge to specific areas-fields, farms, forests, gardens,
roadways, and the like.
Soil mapping itself is an applied science or art. The quality and
usefulness of the result, however, depend upon a vast background
of both fundamental and applied science. They depend upon what
is known generally and upon what is known specifically by the
particular group of scientists doing the work. Every soil survey
area presents a new challenge. It is by no means simply a matter
of mapping a few dozen standard soil types and phases. Soils are
not so easily standardized. The relationships between each soil
and its neighbors, and between each soil and the factors of its
environment, must be sought out and clarified. All likely poten-
tialities for use must be explored and definite judgments arrived
at, insofar as possible, in quantitative terms.
This places a high premium on the resourcefulness of the field
scientist in making full use of all existing data and principles and
in capturing the essentials in the soil-use experience laid out
10 U. S. DEPT. AGRICULTURE IIANDBOOK 18
before him. Nor can the field scientist depend exclusively upon
local sources of information. Important potentialities are sug-
gested from experiences on similar soils elsewhere, even in other
countries. Then the final results must be presented in terms of
adapted crops, management practices, and land use systems, with
awareness of the factors that influence such systems. In short, a
modern soil survey is a difficult research undertaking requiring
intense thoroughness and broad scope.
The rewards of work well done can be very satisfying, both
intellectually and emotionally." Certainly the complexities involved
in understanding a soil and predicting its behavior are enough to,
tantalize the imagination of any man. Then with accurate soil
maps, land users everywhere can make full use of science and
technology to bring forth the great potentialities in the soil,
under efficient management systems, for the sustained abundance
the world so desperately needs.
"For the personal story of a soil surveyor's life in the field, see Macy
H. Laphann's CRISSCROSS TRAILS: NARRATIVE OF A SOIL SUtRVEYOR. 246 pp.,
illus. Berkeley (Calif.) 1949.
CHARACTER OF SOIL MAPS AND REPORTS
A soil mop) is a map designed to show the distribution of soil
types or other soil mapping units in relation to other prominent
physical and cultural features of the earth's surface. The units
may be shown separately or as soil associations named and defined
in terms of taxonomic units. This definition is intended to exclude
maps showing single soil characteristics like texture, slope, depth,
color, or arbitrary combinations or two or more of these; maps
showing soil qualities like fertility or erodibility; or maps show-
ing individual soil genetic factors or combinations of them.
Mapsf of one or more soil features may be made directly from
field observations or by selection and generalization from a soil
map. On a soil map, however, combinations of all observable
features relevant to the nature and behavior of the soil are
comprehended as named taxonomic units-natural bodies with
distinct sets of soil characteristics.
Selected interpretations of soil conditions may be shown on
maps. From a soil map one may derive a series of simple inter-
pretive maps of the same area showing, for example, the relative
adaptability to alfalfa, corn, or other plants, erosion hazards
under defined classes of management, drainage requirements for
optimum production, irrigation potentialities, and many others.
In the making of such generalizations, some soil boundaries are
omitted; for example, those boundaries between soils equal in
erosion hazard on the map of erosion hazard. But these par-
ticular boundaries may be important on another interpretive
map, say one showing productivity classes. Thus different bound-
aries are omitted on different interpretive maps made from the
same soil map.
Most such interpretations are ephemeral. They need to change
with changes in the agricultural arts and in the cultural environ-
ment. If a basic soil map is made accurately, such interpretive
maps can be revised easily from time to time as needed. But if
only "judgment" maps are made on the spot, without a soil map,
with any significant change in the agricultural arts or the cultural
environment all the field work needs to be done over again. In
planning soil surveys, this point can scarcely be overemphasized.
Occasionally "short-cut" rural land surveys are made for some
narrow objective, perhaps at a slightly lower cost than for a basic
soil survey, only to become obsolete in a short time. Such maps
cannot be repaired because vital data were ignored, facts were
mixed with interpretations, boundaries between mapping units
were drawn inaccurately, or because of some combination of
these. Some rural areas have been mapped more than once by
such short-cut surveys at a total cost approximating or even
exceeding that of a basic soil survey and still there is no usable
U. S. DEPT. AGRICULTURE HANDBOOK 18
map for making predictions or recommendations to farmers about
adapted crops, estimated yields, and soil management practices.
A soil map by itself, without a text guide to its interpretation,
cannot be useful to anyone except those soil scientists intimately
acquainted with the units as named in the map legend. To all
others an accompanying text, as well as the map legend, is
essential. The soil survey includes both map and text. In the
text, commonly called the soil survey report, are described the
natural and cultural features of the area surveyed; the charac-
teristics, use capabilities, management requirements, predicted
average crop yields, and predicted long-time effects of manage-
ment systems for each of the soil types, phases, and other mapping
units; and the principal factors responsible for soil formation.
The character and form of soil surveys vary with the soil
conditions, the agricultural potentialities, and the problems to be
dealt with. Also, they have changed over the years with advance-
ments in soil science and in cartographic techniques. Even more
important has been the increased demand for precision in order
to make effective use of the great developments in agricultural
UNITS SHOWN ON THE MAPS
Identification of units.-The first step in making a soil survey
is the establishment of the units of classification to be shown on
the maps. Their nomenclature within the general system of classi-
fication follows their accurate definition, based upon observations
made in the field as supplemented by data from the laboratory.
The basic unit is the natural soil type-the lowest' unit in the
natural (or genetic) system of soil classification. By "natural
system" is meant the system in which all relevant features of soils
are considered as unique interrelated sets of characteristics,
including those important to the practical purposes that soil maps
serve, but without exclusive emphasis upon any one of them.
Each soil type is unique. It is defined as a unique combination
of surface features, like slope and stoniness, and of internal
characteristics-the texture, structure, color, chemical composi-
tion, thickness, and other properties of the horizons that make
up the soil profile to whatever depth is significant. These units
are characterized by field and laboratory observations of the
chemical, physical, biological, and mineralogical features of the
horizons, the geological nature of the parent rock material, and
the geomorphological characteristics of the landscape.
Any one soil type includes the soils that are alike in charac-
teristics that are significant to the nature and functioning of the
soil in the natural landscape. Differences in features that are not
significant in the natural landscape, but which are significant to
'The soil phase as a subdivision of a soil type may be regarded, from some
points of view at least, as a lower unit. But since phases are separated
within soil types, series, families, and great soil groups on the basis of
differences significant (as differentiating soil characteristics) only under
culture and not in the natural landscape, they are not usually regarded
strictly as esse.itial parts of the natural system. (See also p. 289 ct seq.)
SOIL SURVEY MANUAL
the use of the soil in farming, forestry, or grazing are recognized
in subdivisions within the soil type (or soil series). Commonly,
differences in slope, stoniness, or degree of erosion within the soil
type that are not significant in the natural landscape but which
are significant to its use are shown as soil phases. Whereas
soil types are defined within a narrow range of a whole set of
characteristics, including all those of genetic or applied signifi-
cance, phase distinctions within soil types are based wholly on
applied considerations. Thus soil types everywhere should be
defined in the same way; but phases are more narrowly defined
where the agriculture is intensive and less narrowly defined
where it is extensive. The guides to phase distinctions are wholly
In defining the classificational units, including phase distinc-
tions, emphasis is given to the relatively permanent features that
influence response to management and not to ephemeral or transi-
tory features, like the differences in plant nutrients caused by
recent fertilization, liming, or similar soil management practices.
It must be recognized that the immediate productivity of areas of
the same soil type, or phase, may vary because of recent manage-
ment history. This is especially true of soil types that respond
greatly to fertilizers. Nevertheless, there should not be significant
differences in productivity for climatically adapted crops among
areas of the same kind of soil, if properly mapped, when given
the same management. In very old agricultural areas, however,
practices have changed the soils fundamentally, and their
Observable features and inferred qualities.-In carrying out the
soil survey and in reporting the results, the observable features
need to be clearly distinguished from those soil qualities that
are learned only by inference.
In the completed soil survey, the features of each kind of soil
are listed. Among those observed directly are slope (degree, shape,
and pattern), stoniness, depth, and the color, structure, texture,
and other significant features of each horizon of the soil profile.
Other observations include soil temperatures, kinds of plants and
their rooting habits, features caused by erosion, and so on. Many
characteristics are determined partly through the use of scientific
instruments. Among these are the contents of clay, organic
matter, plant nutrients, exchangeable cations, and the various
clay minerals in the soil horizons. The pH of each soil horizon
is also determined. As needed, the degree of aggregation, perme-
ability, kind and amount of soluble salts, and the effects of addi-
tions of water are determined. It may be emphasized again that
the soil units may be grouped and interpretive maps made
according to one of these observable characteristics, but such
maps are not basic soil maps.
Through interpretation from observed features, the qualities of
kinds of soil may be learned by inference. Soil fertility, for
example, may be estimated from observable characteristics, from
the results of experimental plots, and from the experiences of
U. S. DEPT. AGRICULTURE HANDBOOK 18
farmers having records on fields consisting largely of one kind
of soil. Soil fertility, however, is not directly observable. It is the
quality that enables the soil to provide the proper compounds, in
the proper amounts and in the proper balance, for the growth
of specified plants, when other factors, such as light, temperature,
moisture, and the physical condition of the soil, are favorable.
Thus soils may be grouped into fertility classes only by inference.
The same is true of tilth-the physical condition of the soil in
respect to its fitness for the growth of a specified plant. Combining
both of these qualities, fertility and tilth, one arrives at the
concept of productivity, defined as the capability of the soil for
producing a specified plant or sequence of plants under a specified
set of management practices.
Groupings of soils by inferred qualities are essential to the inter-
pretation of a soil survey. Besides fertility, tilth, and produc-
tivity, several other qualities may be inferred from the basic soil
survey if the research is carried on competently. These qualities
include erodibility, irrigability, response to drainage, workability
or physical condition in respect to tillage, and crop adaptability.
Groupings of soils according to use capability, either in the
general sense or in the special sense employed by the Soil Con-
servation Service in its program of assistance to farmers, are
easily made from the detailed soil map and report, or can be
read directly from the soil map.
Identification of boundaries.-Having established the units of
classification and identified these units on the ground, boundaries
are drawn among them on accurate base maps or aerial photo-
graphs. The scales to be used depend upon the uses to be made of
the map and the relative intricacy of the soil pattern.
After the soil units have been defined and their relationships to
the environment worked out, most soil boundaries can be located
on the land surface by recognizing where changes in one or more
of the genetic factors occur. That is, excavations or borings are
needed chiefly to identify the profile of a soil landscape. The
actual boundary can usually be drawn most accurately by careful
observations of the landscape. Nonetheless, there are important
exceptions where the relationships between the differentiating
soil features and genetic factors are obscure. For example, the
depth and thickness of an iron crust or of a horizon of carbonate
accumulation, or the depth to a water table, may be variable
although there is no corresponding variation in surface features.
In such instances, examinations of the soil are necessary for
locating boundaries as well as for identification.
Soil boundaries must be drawn accurately. Despite the large
proportion of attention given to soil classification in contrast to
methods of soil mapping, a large part of the poor soil maps in the
world are poor mainly because of inaccurate boundaries-boun-
daries guessed at rather than determined. In soil survey work
great emphasis must be given to honesty in research. It is more
difficult to check the results of a soil mapper than to check those
SOIL SURVEY MANUAL
of a laboratory worker, and the damage from incorrect soil
boundaries may be very serious to the map user.
KINDS OF SOIL MAPS
Depending upon the detail with which boundaries between the
mapping units are plotted in the field, three general kinds of
original soil maps are recognized: (1) Detailed, (2) reconnais-
sance, and (3) detailed-reconnaissance. Of these the detailed soil
survey is the most useful and most important. The third, detailed-
reconnaissance, is not really a separate kind but is a soil map
having parts of each of the first two kinds.
Besides original soil maps made from field surveys, there are
relatively small scale soil maps showing associations of the taxo-
nomic units. Gencralizcd soil maps are developed through orderly
abstraction from original field surveys, either detailed or recon-
naissance. Schematic soil maps are compiled from spot field obser-
vations of the soils and their genetic factors, and from maps of
geology, climate, land form, vegetation, and relief. Generalized
soil maps of representative areas guide the compilation of schem-
atic maps and are usually included in some parts of them.
Detailed soil maps.-On a modern detailed soil map, the soil
types and phases are mapped in the detail required to show all
boundaries between mapping units, including areas of one unit
within another, that are significant to potential use (generally to
plan field management systems). The classificational units are
defined narrowly enough to be homogeneous genetically and to
permit making such significant differential predictions as avail-
able knowledge permits; and the boundaries between mapping
units are plotted on base maps or aerial photographs from obser-
vations made throughout their course, along with such natural
features as streams and lakes and such significant cultural fea-
tures as ditches, roads, railways, and houses.
Specific guides on the many items are presented elsewhere in
this Ma o(al. The base map needs to be complete and accurate be-
cause land lines (section lines, township boundaries, and the like),
roads, houses, streams, and other obvious features are needed as
local i. i. i.: points by map users. Great detail in soil mapping,
without a detailed base, is largely wasted, since the user is usually
unable to locate himself properly and read the map accurately.
Accuracy of a soil map is therefore not determined primarily by
general geodetic accuracy but by what might be called local
accuracy-the relation of the soil boundaries to the other features
that the map user can identify. For example, even though a soil
boundary may be plotted within the general limits of accuracy,
should it be on the wrong side of a house or road, the usefulness
of the map and the user's confidence in it are greatly reduced.
The detail of boundaries required depends partly upon the pros-
pective use of the map. If small bodies of one kind of soil occur
within areas of another kind of soil and thereby significantly
affect management, the small bodies of soil should be separated or
indicated on the map by defined symbols, even if they are an acre
U. S. DEPT. AGRICULTURE HANDBOOK 18
or less in extent. Judgment in mapping such areas is also influ-
enced by the relative contrast between the two kinds of soil.
Even with large map scales some taxonomic units are often so
intricately interlaced with one or more others that the association
of them needs to be recognized as the mapping unit. In mapping
areas of complex patterns where all the soils contained in pad-
docks or fields are treated alike, it may be more useful to show
well-defined soil complexes than to map individual taxonomic
units in minute and intricate detail.
The scale of mapping depends upon the purpose to be served,
the intensity of soil use, the pattern of soils, and the scale of
other cartographic materials available. Commonly a scale of 4
inches equal 1 mile (1:15,840) is now used for field mapping and
one of about 2 inches equal 1 mile (1:31,680) for publication.
Few detailed soil surveys that meet modern standards can be
made in field scales less than 1:20,000 except in comparatively
uniform terrain. For planning irrigation developments and in
areas of very intensive farming the field mapping scales may need
to be larger, say 1:7,920 or even 1:5,000. For engineering work,
like planning for highway or airport construction, the detail
needed may require a field mapping scale of around 1:2,500 or
In former years many soil maps were made in the field at a
scale of 1 inch equals 1 mile (1:63,360 or 1:62,500) and published
at the same scale. Later, the field mapping scale was doubled to
1:31,680. After the use of aerial photographs became general, the
field scale was increased again to around 1:20,000 or 1:15,840.
Publication scale continued for some time at 1: 63,360 or 1: 62,500.
Some of the detailed maps plotted in the field on aerial photo-
graphs and reduced to these scales in publication are extremely
difficult to read. So the publication scale was later increased to
1:48,000 and then again to 1:31,680 or 1:24,000. These larger
scales have become necessary for easy legibility, although broad
geographic relations among the soils are less clearly seen with the
reduced total area of land on a single map sheet. The advantage of
having the detailed soil survey of a county on one single map, how-
ever, has had to be sacrificed for clear reading of detail in refer-
ence to individual fields and farms.
No general rule can be laid down for guiding the number of
soil examinations required per unit area nor for the intervals
between traverses, except that these can rarely be more than one-
fourth mile wide and usually need to be narrower.
Reconnaissance soil maps.-On a reconnaissance soil map the
boundaries between the mapping units are plotted from observa-
tions made at intervals and not necessarily throughout their
whole course as on the detailed soil maps. Reconnaissance maps
vary widely, from "semidetailed" soil maps that approach the
specifications of a detailed soil survey to maps of soil associations
made from traverses at intervals of several miles. Reconnaissance
maps are usually planned for exploratory purposes-to discover
and outline areas of soil suitable for more intensive development
SOIL SURVEY MANUAL
(see page 435 et seq.). They are particularly useful in new and
relatively undeveloped regions for identifying areas of promise for
settlement or more intensive use.
In some reconnaissance surveys, the classification units are less
precisely defined than in detailed soil surveys. Usually the map-
ping scale is smaller and fewer mapping units can correspond to
the taxonomic units. In older reconnaissance work, it was cus-
tomary to show named soil types on the map for areas that were
really undefined mixtures of that soil type with others. This was
done especially where research was insufficient to develop a com-
plete classification. It is now possible to make far better and more
useful maps by using defined soil associations.
In modern reconnaissance mapping, the taxonomic units are
sought out, defined, and named as in a detailed soil survey. These
are then mapped in groups as geographic associations. Such an
association may contain several sharply contrasting soil types
and phases. Each association is defined in terms of the named
taxonomic units, their relative proportion, and their pattern. The
associations are named in terms of the more prominent taxonomic
During the progress of the work, representative sample areas
of each soil association are mapped in the detail required to meet
the specifications of a detailed soil survey. These areas are care-
fully located, and small maps showing the detail are reproduced
separately in the accompanying text. The usual supplemental lab-
oratory data and other data are assembled by taxonomic units.
Predictions about adapted crops, estimated yields, management
requirements, and so on are also made for these units as they are
in the detailed soil survey.
Agricultural scientists and advisers can examine the sample
areas and learn how to identify the individual taxonomic units
within the particular soil associations that concern them.
This scheme of reconnaissance soil mapping has a wide applica-
tion in new and relatively undeveloped areas. It makes possible
better appraisals of regional potentialities than the older recon-
naissance soil maps with poorly defined mapped units. It permits
the rapid surveying of large areas where development cannot
await the completion of a detailed soil survey. At the same time
it gives advisory agriculturists an opportunity to make those
specific recommendations that can only be made on the basis of
local, narrowly defined soil types and phases.
Good reconnaissance maps can be made only if there is enough
detailed mapping of representative sample areas to establish the
modal definitions of the taxonomic units and their permissible
ranges of variability. Specifications for individual maps will vary
widely. In mountainous regions or other areas not likely to be used
intensively, traverses are made at less frequent intervals than on
land suitable for farming.
U. S. DEPT. AGRICULTURE HANDBOOK 18
Many of the soil maps made in the earlier years of research in
the United States, which were looked upon as detailed soil maps
in terms of the techniques of that time, are regarded as reconnais-
sance soil maps under modern specifications. The increased detail
did not come in any single year and there were wide variations in
the skill and vision of individual supervisors and soil survey
Detailed-reconnaissance soil maps.-On a detailed-reconnais-
sance map some portions satisfy the specifications for detailed
soil maps, whereas other portions are reconnaissance soil maps.
Such maps are made of counties or other geographic units con-
taining areas of soil used or potentially useful for agriculture
and other large areas that are unsuited. The part covered by re-
connaissance may be rough mountainous land, raw acid peat soils,
stony desert soils, dry sandy plains or hills, or other landscapes
unsuited to farming.
The boundaries between the detailed and reconnaissance types
of survey on the one map may be made in one of two ways: (1)
The boundaries may follow section lines or other land lines and
be shown in a smaller sketch map on the margin of the soil map;
(2) the legend on the map may be divided into two parts. The
mapping units listed under the reconnaissance legend, and bound-
aries among them, are defined and mapped according to the speci-
fications for the reconnaissance map; whereas those units listed
under the detailed legend and boundaries among them and between
them and the units listed under the reconnaissance legend, are
mapped according to the specifications of the detailed soil map.
Where the area covered by reconnaissance is considerably
larger than the area covered in detail, it may be convenient to
publish the reconnaissance portion separately from the detailed
mapping. The detailed maps can then be published on extra sheets
at a larger scale.
Generalized soil maps.-In order to see the broad geographic re-
lations among soils, small-scale maps are necessary to bring out
the contrasts among regions. The best of these are generalized
from detailed soil surveys. Such maps vary in scale and detail
from soil association maps of counties at a scale of 1 inch equals
1 mile (1:63,360) to single maps of large regions showing associ-
ations dominated by one or more great soil groups.
The descriptive legends of soil association maps indicate the
relative proportions and patterns of the several classificational units
that compose them. If the map is included as a part of a detailed
soil survey, the text that explains the individual taxonomic units
on the detailed soil map can serve for both. If the soil association
map is published separately, descriptions and predictions for all
taxonomic units within the associations should be attached, per-
haps in tabular form.
SOIL SURVEY MANUAL
The publication of detailed soil maps at scales as large as
1:31,680 and 1:24,000 has increased the need for generalized
soil association maps so that broad areas can be viewed as a
whole. Since the county is a convenient unit for many kinds of
agricultural work in the United States, a soil map is needed to
exhibit the whole county in such a way that the various parts of
it stand out according to the principal soil features and patterns
that are basic to types of farming and community problems.
Since the uses of generalized soil maps are so varied, it is more
difficult to write specifications for them than for detailed soil
maps. For the lowest level of generalization, the one most useful
in agricultural advisory programs, we may proceed on the follow-
ing basis: Farms are usually made up of several soil types. It is the
combination of soil types that gives the soil association its dis-
tinctive character and sets the potentialities and limitations within
the farm unit. Experience on individual fields is synthesized,
classified, and extended on the basis of soil types and phases as
defined in the detailed soil survey. Experience with whole farm
units, made up of combinations of soil types, is synthesized, classi-
fied, and extended on the basis of soil associations. Consequently,
the legend and detail of a useful soil association map are planned
to show the use suitabilities of these broad geographic groups of
soils. Of course, boundaries between soil associations cross some
farms, just as the soil type boundaries cross some fields. If large
areas of a single soil type do dominate many whole farms, the
soil type may be shown separately. But rarely is this possible. In
recognizing very small strips of highly productive Alluvial soils,
for example, it must be recalled that the Aluvial soil usually is
only part of the farm and is used in association with the adjacent
uplands. In such instances, it may be misleading to separate small
strips of Alluvial soils as a distinct association and the upland
soils as one or more others. In other words, excessive detail in the
soil association map can lower its usefulness.
The development of a proper legend for such a generalized soil
association map requires judgment based upon a study of both
soils and farming systems in whole farm units. The form of the
legend for a soil association map is influenced by cultural environ-
ment, or expected cultural environment in a new area, more
than is that for a detailed soil map. It must be so influenced if the
soil association map is to be most useful for indicating whole-
farm and community problems and potentialities. Well-made soil
association maps interpreted in the light of data from experi-
mental plots, fields, and farms, are exceedingly valuable for
classifying farms according to their basic potentialities and for
guiding agricultural advisers in the geographic emphasis they
should give within a county or district to various educational and
demonstration programs. Soil association maps serve as an ex-
cellent basis, in fact the only satisfactory one, for suggesting the
approximate locations of experimental farms, pilot-research
U. S. DEPT. AGRICULTURE HANDBOOK 18
farms, and demonstration farms'-, and for suggesting where the
experience from these farms is most applicable. For the exact
location and plans of such farms a detailed survey is required.
Soil association maps indicate the areas where the agricultural
adviser should emphasize liming, erosion control, drainage, forest
planting, use of phosphatic fertilizers, expansion of pastures, and
like practices or combinations of them.
Still smaller scale soil association maps of States or regions are
useful in assisting the advisers in community development.; On
these, the smallest land area to claim attention is larger than a
farm, generally about the minimum size for a homogeneous agri-
Schematic soil maps.-In form and appearance these resemble
generalized maps of soil associations. Scales are usually small,
say 1:1,000,000 or smaller, although useful ones are made at
larger scales. For many areas, especially in new and undeveloped
regions, it is useful to have an approximate or estimated soil map
even in advance of an organized field soil survey, either reconnais-
sance or detailed. Such maps may be made by estimating the
soil pattern. If carefully (lone by highly competent scientists, this
is a great deal more than guessing.
First, all available data, both at spots and in map form, on the
soils and the climate, vegetation, geology, and land form, are
gathered and studied. In wild areas, these data may consist
mainly of notes taken by scientific travelers and rough maps
made from aerial photographs without proper ground control. A
soil is the unique result of five interrelated factors: (1) Climate
and (2) living matter, as conditioned by (3) relief, acting on
(4) parent rock materials for periods of (5) time. Therefore, if
reasonably good estimates can be had of all but one of these fac-
tors, the missing one may be interpreted by geographic correla-
tion. This is the principle. If good topographic maps are available,
often surprisingly good soil maps can be forecast by experienced
soil scientists thoroughly familiar with the combinations of envi-
ronmental factors that produce different kinds of soil.
Since the amount and reliability of available data vary greatly
from place to place, schematic soil maps always need to be accom-
panied by a sketch map showing relative reliability.
SAn cxpcrhrimntal farm is one on which experiments are conducted on
single enterprises without regard to the farm unit as a whole, say plot studies
of fertilizers, crop varieties, and rotations, or pasture experiments with
grazing animals. On a pilot-irscarclh farm the aim is to find the optimum
combination (or combinations) of practices suited to the farm as a unit. Both
the experimental and pilot-research farms are managed for research results,
and decisions are made by the scientists in charge. On the demonstration
farms, proved practices are applied mainly by concentrating advisory serv-
ices to help the olwcator make the best decisions possible toward optimum
farm and home development. Predi'('cilojiint farms, part way between
pilot-research and demonstration farms, are sometimes established a few
years in advance of settlement as guides to the new settlers.
'For an example of this use see MONT(GOMERY COUNTY [Alabama] FARM
PROGRAM. Agricultural Extension Office, Montgomery, Ala., (;t pp. (c. 1947.)
SOIL SURVEY MANUAL
The interpretation and use of schematic soil maps for agricul-
tural and engineering purposes follow the same course as for
generalized maps. The soil associations need to be defined accord-
ing to the taxonomic units that compose them, their proportions,
and their patterns. Then the characteristics and predictions may
be given for the individual taxonomic units insofar as they can be
estimated; and soil potentialities and problems for community
development may be given for whole soil associations. Commonly
it is not possible to go further down the scale in the taxonomic
classification than great soil groups, with subdivisions according
to parent rock, slope, depth, and stoniness.
The compilation of a schematic soil map is often the first logical
step in planning more detailed study and survey of a large unde-
veloped area. After compilation of the schematic soil-association
map, representative sample areas may be mapped in detail. Keys
and tables of predictions for the local soil types and phases within
each soil association can be worked out. After the sample areas
have been mapped in detail, the approximate schematic map first
drafted can be revised. The schematic map can then be published,
along with the detailed sample maps and their explanations, as
a useful guide for appraising the potentialities of the various
parts of the region. The published survey should include specific
guides that will enable agricultural advisers to recognize local
soil types and phases, for these will aid them in making specific
recommendations to soil users.
Exploratory soil maps.-These maps resemble schematic soil
maps except that the mapping units are identified mainly by
original observations of soils within the area, even though the
boundaries are largely compiled from other sources.'
REQUIREMENTS FOR THE SOIL SURVEY REPORT
The report, or text accompanying the soil map, is an essential
part of the soil survey. Since its form and content depend upon
the purposes to be served, these must be thoroughly understood
in advance. The report is not an extra chore to be (lone after the
map is made; it needs to be developed along with the mapping in
the field. For a basic general-purpose soil survey, a complete state-
ment of all essential soil characteristics and their variabilities
needs to be included, regardless of the immediate practical needs
to be served. The soil scientists in the field need to know as much
as possible about the probable uses; but they must also not be
prejudiced by these to the point of omitting significant soil char-
acteristics because they seem relatively unimportant at the
moment. Time and time again, soil surveys have been found to be
very useful indeed for purposes never dreamed of by the soil
survey party doing the original field work. If the essential facts
were recorded, the maps could be interpreted readily for the new
purpose; otherwise, the field work had to be done over again.
'As one example see K(ELLOc, CCHARLES E., and NYvAun, IVERi J. EX-
PLORATORY STUDY OF THE PRINCIPAL SOIL GROUPS OF ALASKA. U. S. Dept.
Agr. Agr. Monog. No. 7, 138 pp., illus (map). 1951: Washington, D. C.
22 U. S. DEPT. AGRICULTURE HANDBOOK 18
The uses of the soil survey are expanding so much that more
than one report is sometimes necessary. For the lay reader, ex-
planation of interpretations as they relate to his immediate prob-
lems may be all that is required. This may be included in the basic
report or published separately. Such statements may need to be
revised from time to time with changes in the agricultural arts
and in economic conditions, and issued as supplements. Then too,
special reports on engineering features or other interpretations
may be necessary. Ordinarily, the publication of such special
reports in the United States is a responsibility of the cooperating
local research institute, like the State agricultural experiment sta-
tion, rather than the Federal Soil Survey.
Normally, as the work progresses, the soil survey report grows
out of the descriptive soil legend. The soil descriptions are already
complete when the mapping is finished. The available geological,
climatic, and agricultural data are obtained in advance of the
field mapping, for they are useful in developing the descriptive
legend and guiding the taking of field notes.
PURPOSE OF SOIL MAPS AND REPORTS
The Soil Survey includes those researches necessary (1) to de-
termine the important characteristics of soils, (2) to classify soils
into defined types and other classificational units, (3) to establish
and to plot on maps the boundaries among kinds of soil, and (4)
to correlate and to predict the adaptability of soils to various
crops, grasses, and trees, their behavior and productivity under
different management systems, and the yields of adapted crops
under defined sets of management practices.
The fundamental purpose of a soil survey, like that of any
other research, is to make predictions. Although the results of
soil research are being applied increasingly to engineering prob-
lems, such as the design and maintenance of highways, airports,
and pipelines, applications are chiefly in the agricultural field.
including forestry and grazing. It is purposeful research.
The many thousands of different kinds of soil have unlike
management requirements for economic, sustained production.
For centuries farm families learned as best they could through
trial and error what methods worked best on their various fields.
This knowledge passed on from father to son, but it could not be
transferred readily to other areas, nor could the experience on
other farms be applied safely.
With the development of modern science, agriculture is being
made continually more efficient. Progress has been phenomenal
during the 50 years since soil surveying began in the United
States. Even the rate at which agricultural efficiency is being in-
creased is itself accelerating as this Manual is being written. Ex-
periments with soils, plants, and animals are being continued in
many parts of the world. New farming systems are being tested
in both research and practice. Fundamentally, soil classification
serves as the basis for classifying, synthesizing, and reporting
these results of research and experience. The more agricultural
science progresses, the more important this work becomes. The
investments in machinery and materials per acre of cultivated land
are increasing. The planning of farm systems for optimum sus-
tained production needs to be done far in advance of the operations
for the best results made possible by modern science, with re-
visions from season to season. The importance of precise recom-
mendations-differential recommendations from field to field and
from farm to farm-increases. Soil maps serve as the basis for
such differential recommendations.
Crop plants and soil management practices are so sensitive to
the differences in soil that a soil survey adequate for this basic
need is certain to serve a great many other purposes as well. In
fact, no other maps of large areas of land are made in such detail
and involve so many significant factors as do soil maps.
U. S. DEPT. AGRICULTURE HANDBOOK 18
SYNTHESIS OF AGRICULTURAL DATA FOR APPLICATION
TO SPECIFIC AREAS
The soil survey is an integral part of an effective agricultural
research and advisory program. It is clearly impossible to carry
out exhaustive and expensive researches on every field and farm.
Representative samples of land must be chosen. The soil type or
phase, accurately defined and named in a standard system of
classification, is the only reliable basis yet found for selecting
such samples. Every experimental plot is a sample of a landscape.
It should be an accurate and representative sample of a kind of
soil worth sampling. Thus the soil survey has an important r6le
in the planning of research, especially in the selection and location
of experimental fields and farms.
New discoveries from experimental work and on farms need
to be extended to other areas of similar soils. For optimum use,
new methods must be tested widely in farming systems. As the
new discoveries are tested, the results can be classified by kinds
of soil. When we know that a certain soil area is Miami silt loam,
let us say, a great body of research and farm experience is avail-
able to allow us to predict its management requirements, the
crops that may be grown and their yields, and the long-time effect
of various management systems on its productivity.
Without the results of a large amount of correlative research
and of careful farm analyses to help them, the scientists in soil
survey will be unable to give good predictions. Contrariwise, it is
through the soil survey that the results of a host of other re-
searches can be precisely applied.
Through study and comparison of soil types and phases which
are defined as sets of soil characteristics, of the sets of genetic
factors that go with them, and of the synthesized results of farm
analyses and correlative research, general principles of soil be-
havior are developed for various levels of soil groupings. In going
to the higher categorical levels of classification, from soil type to
series, to families, to great soil groups, and finally to suborders,
the number and precision of the generalizations are reduced.
For detailed predictions and recommendations, the soil type,
or a phase of a soil type, is the safest base because of the narrow
range of characteristics. If all possible interpretations are to be
given, it is the only possible base. But for some one interpretation,
as response to liming or the erosion hazard, several soil types and
phases can be grouped together.
It should be emphasized that soil scientists, acting strictly as
soil scientists, give predictions rather than recommendations.
The prediction statements and tables in a soil survey report are
designed to predict the results from using the soil types or phases
in various defined ways. But the alternative to be recommended
for a specific operating farm depends upon the economic environ-
ment of the farm and the skill, facilities, and desires of the oper-
ator. Then too, for most soils several combinations of practices
SOIL SURVEY MANUAL
Given an accurate soil map of a farm, alternative cropping and
soil management systems for that farm may be developed from
the predictions given. With competent soil survey work, with
predictions about the other production factors-livestock feeding,
performance of machinery, disease protection, and the like-and
with adequate consideration of the economic factors, optimum
farming systems can be developed.' Clear statements of the alter-
natives are necessary so that agricultural advisers and farm
operators can make proper selections from among them.
Since the decisions about farming practices are made within
millions and millions of individual managerial units, classification
must be detailed enough to include all the significant soil charac-
teristics-all basic land features that significantly affect soil use
and management. The maps must be detailed enough to indicate
areas of soils significant to a farm management system. They
must show these areas accurately in relation to local reference
points shown on the map that the user may recognize on the
Increasingly, the results of the soil survey and of the correlative
research are applied by the farmer, often with some advice,
through the development of a farm plan. Such a plan to be useful
does not need to be elaborate. In addition to the use of each field,
it shows field boundaries, alternative boundaries, and more or less
permanent structures, such as buildings, fences, drainage and
irrigation canals, terraces, waterways, and the like. The soil
boundaries may be obtained from the soil map. A few of these may
coincide with certain field boundaries. In fact, a major contribu-
tion of soil mapping to farm planning is the help it gives in relo-
cating field boundaries in order to make fields more nearly
uniform. A field containing one kind of soil can be handled more
effectively than one containing two or more contrasting soils.
The use of the several fields should be indicated tentatively as
far in advance as practicable, with alternative cropping systems,
so that shifts can be made with unusual weather or with signifi-
cant changes in economic conditions.
A good farm plan is carried beyond the field layout and cropping
system to a farm budget. Such a budget is very important as a
test against the physical layout. Farm plans that have called for
drastic changes have often failed unless first tested against an
estimated budget. To make a budget, at least rough inventories
are required of carry-over feeds, machinery, and livestock." For
most farms, several alternative plans, with budgets, may be cal-
culated, any one of which will maintain and improve the soils.
'For a discussion of the development of optimum farming systems, see
BLACK, JOHN D., ct al. FARM MANAGEMENT. 1,073 pp., illus. New York.
See JOIINSON, NEIL W., and BARNES, C. P. PLANNING FARM RETURNS.
U. S. Dept. Agr. Yearbook 19.13-47. (Science in Farming): 905-910, Wash-
ington, I1947; and also Black, J. D., ct al. in the General Bibliography.
U. S. DEPT. AGRICULTURE HIANDBOOK 18
The one chosen depends upon the skill, resources, and likes of the
It is unnecessary here to go into a detailed explanation of farm
planning except to point out its requirements so that those making
detailed soil surveys can make sure their work will be satisfactory
for the purpose. In planning, the farmer and his adviser should
consider the enterprise combinations that are adapted to the farm
as a whole, their economic feasibility, and the skills, resources,
and desires of the farm family. No matter how listed, all phases of
soil use and farming practices are interrelated. With that in mind,
the following is a check list of the principal elements in the farm
plan for sustained production that depend wholly or partly upon
a proper interpretation of the soil conditions that are taken into
account in soil classification and mapping and in the soil survey
1. 1Major land ises.-The plan needs to be balanced among the
major land uses-crops requiring tillage, forestry, and pasture-
according to the pattern of soil types on the farm and the require-
ments for balance among the several enterprises. Where livestock
is produced, the farm needs a proper balance between pasture
and feed crops. The several farm operations have to be balanced
in relation to the labor supply. Provision needs to be made for
the home orchard and garden where practicable.
2. C,..,',..':.t system)f.-A well-planned cropping system is needed
that fits the kinds of soil on the farm. Usually crops should be
grown in rotations or mixed cultures. Good seed of those varieties
having the greatest disease resistance, drought tolerance, yield,
and quality should be used. Most soils produce best with crop
rotations that include meadows having deeply rooted legumes or
3. T:lii,:, methods.-The methods employed in tillage should be
aimed to prepare seedbeds properly and on time, to make the soil
receptive to water, to incorporate organic material, lime, and
fertilizer deeply where necessary, and to control weeds. Where
soil blowing is a hazard, the surface must be left cloddy and
trashy. Many good machines are available from which selections
can be made. On some soil types, the moldboard plow, or turning
plow, is best; on others, it should not be used.
4. Protection.-Both crops and livestock should be given the
necessary protection against winds, insects, and other hazards.
It is often important to know whether or not the soil can be used
for growing shelter belts.
5. Water control, Ius, and disposal on, the land.-Every farm
needs an orderly system of water use and disposal. Many farms
have naturally well-drained soils and dependable rainfall. A large
number do not. Excess runoff of rain water must be reduced to
the minimum with protective close-growing plants, strip crop-
ping, terracing, or in other ways, so that the water will soak into
the soil for plant growth and not be lost or cause erosion. On
SOIL SURVEY MANUAL
erodible soils where rains are intense, unless the management
plan provides for runoff and erosion control, all other practices
may come to nothing. Although the amount of erosion that has
already taken place is significant, the important thing is to assess
the hazard of erosion, whether or not much has taken place. Some
soils need drainage. Low lands need protection from floodwaters.
Many soils will respond to irrigation. Some of these practices
require community effort, but a lot can be done by the farm
6. Use and conscrvation of organic matter.-Large and unneces-
sary losses of animal manure and crop residues often take place
through fire, leaching, and neglect. Yet many soils respond
enormously to the addition of organic matter. Part of the need
for soil organic matter can be met in a cropping system itself
by using a grass-legume mixture, deeply rooted legumes, green-
manure crops, and cover crops.
7. Reaction control.-On acid soils liming is a first essential to
create soil conditions favorable for the availability of the other
plant nutrients and for the deeply rooted legumes. In the regions
of low rainfall, provisions are required for eliminating excess salt
or alkali and for preventing their accumulation under irrigation.
8. Fertilization.-A system of fertilization may need to be
developed in the farm plan to make possible the best combination
of high-yielding crops. We must always recall that fertilization
may offer an excellent opportunity to expand the choice of crops
that may be grown. One cannot recommend the precise amounts
of fertilizer to use from the soil map alone; other aspects of the
farming system already mentioned and previous use must be
considered. For both lime and fertilizer recommendations, it is
helpful to have the results of appropriate chemical tests in areas
where reliable ones have been developed. The reader should be
able to interpret from the soil map and report, however, the
general fertilizer requirements and the production that may be
expected from systems involving their use; but the need for
phosphorus, say, on any one field will depend also on the amounts
that have been used in former years and on other phases of the
These aspects of farm planning are so clearly interrelated that
decisions about one influence the others. The crop rotations, for
example, depend on liming and fertilizing and the erosion hazard;
the nitrogen fertilizer required depends partly on the legumes
grown and the manure applied; and so on.
Farm classification can be a great aid to advisory work and to
farm planning, especially where the soils, and the optimum sets
of practices to go with them, are contrasting. With a detailed soil
map and the pattern of farm boundaries, farms may be grouped
according to amounts and kinds of soil resources into classes of
farms having similar potentialities and problems. The need for
this kind of farm classification is greatest in areas where the
local variations among soils are greatest. In Iowa, for example,
28 U. S. DEPT. AGRICULTURE HANDBOOK 18
the need is less striking than in a State like Tennessee, where
the local soil variations are many and great.
RURAL LAND CLASSIFICATION
The results of the soil survey are often applied through an
intermediate grouping of the soil types and phases, often called
"land classification." The soil units shown on the map may be
grouped into classes on any one of several bases, such as (1)
degree of some characteristic like texture, stoniness, slope, or
acidity; (2) adaptability to some crop or group of crops; (3)
productivity under certain sets of management practices; (4)
erosion hazard and general management requirements for erosion
control; (5) potential irrigability; and (6) response to lime,
phosphate, potash, or other fertilizers. It is clearer to call group-
ings like these "soil groups" than to label them "land classes," in
order to avoid the broad connotation of the word "land."
The data of the soil survey are often used to classify, for
various purposes, specific geographically defined bodies of land,
like sections, "forties," or farms, as shown in a cadastral survey.
A clear distinction is needed between the classification of specific
land tracts-sections, lots, or other cadastral subdivisions-
perhaps more aptly referred to as land classification, and the
classification of land into kinds, types, or classes irrespective of
cadastral or property boundaries. In the former, distance from
market, size of tract, and other relevant factors of the institu-
tional environment can be evaluated with some accuracy; whereas
distance from market or size of area are not relevant in grouping
the soils, let us say, according to productivity for adapted crops,
except as the general social and economic environment fix the
perimeter within which the groupings need to be made. No one
recommends, for example, that research be undertaken now to
find the productivity of soils in Maryland for paddy rice, nor of
those producing sugarcane in Hawaii for buckwheat or rye.
Multiformity of land classes.-In a sense the soil survey may
be called a kind of land classification. Although it does not include
all the characteristics of place, it certainly recognizes a larger
proportion of the ones relevant to local land use, and more accu-
rately, than any other survey systematically carried over large
areas. But as the term "land classification" has been most com-
monly used, it usually refers to something far less complete and
detailed than a modern soil survey.
The term "land classification" can easily become very confusing.
The attributes of any area are exceedingly numerous, and their
relevance varies enormously in different parts of the world; yet
any one or any combination of the attributes may be chosen as
criteria for a "land classification." The matter is even worse than
that. Many "land classifications" are more or less personal inter-
pretations of undefined combinations of attributes, economic
appraisals, or use experiences, often in relationship to a shifting
undefined standard. Lands have been classified using tax delin-
quency, condition of farm buildings, growing vegetation, intensity
SOIL SURVEY MANUAL
of use, patterns of use, and so on as criteria of use capability or
other land qualities, in both meticulous detail and broad sweeps.
Many of these classifications have been useful but some have been
misleading indeed, partly because ephemeral standards were used
in the work and especially because factors relevant and vital to
the purpose of the classification were not taken into account.
The misuse of land classifications often comes about by shifting
a fixed method from one soil or cultural region to another. For
example, in some areas a general relationship has been found
between soil quality for farming and tax delinquency, partly
because of the common overassessment of unresponsive soils. In
such an area, land classification based primarily on tax delinquency
and whether or not land is cleared may give a workable basis
for rural zoning. A similar classification fails badly, however,
in an area where there is plenty of labor for clearing land, or
where unresponsive soil is not overassessed. In some soil regions
an exceedingly close relationship exists between native vegetation
and soil groups based on the productivity of the kinds of soil for
cultivated crops. Yet possibly only 100 miles away, with a slight
difference in climate, the "good indicator" species push well over
onto soils unsuited to farming. Since plants grow as a result of
a combination of growing conditions, they cannot be taken as a
certain evidence of either climate or soil. Examples of similar
errors could be multiplied many times.
Land classifications based mainly on present land use are per-
haps the most likely to mislead. Yet they can be very useful, too.
Many institutional, economic, and historical factors, besides soil
productivity, have combined to determine present use. Intensive
use does not necessarily indicate highly responsive soils, adapted
crops, nor optimum farming systems. Large areas of responsive
soils in the world remain largely unused because of lack of
transport or industry, or from the accidents of colonization; but
land-use maps, especially where intensity of use can be inter-
preted from the maps, can be useful as a supplement to the soil
map. By comparing the two maps, one may ascertain what users
have found to be possible and what areas are used with less than
the possible intensity. Such comparisons give a beginning point
for searching out the obstacles to optimum soil use, many of
which may turn out to be economic or institutional.
Another source of confusion in land classification to many
has been the search for a simple, all-purpose classification of
land according to its characteristics and capabilities. This the
authors now regard as an impossibility, despite hopes expressed
in the first edition of this iMainal and elsewhere. If the classifica-
tion is simple, relevant factors must be omitted. The number of
significantly different soil series runs into the thousands for the
continental United States alone. There are even more in the
tropics. Then, when we add to these the necessary phase distinc-
tions for variations within soil types, the number of kinds of soil
becomes much larger. Besides soil, as defined in this Manil,
there are climatic variations that are significant to growing
30 U. S. DEPT. AGRICULTURE HANDBOOK 18
plants within the environment even of some soil types. All
sorts of variations in vegetation may be expected. Thus the
classification cannot be simple except for an easily defined, narrow,
single purpose. As already explained, it is generally far cheaper
to make a basic soil survey from which a great many simple
groupings, or "land classifications," may be derived by interpreta-
tion, than to concentrate on one narrow immediate objective at
a time in separate surveys.
Nor can there be an all-purpose classification or grouping. A
grouping made primarily to indicate erosion hazard and for
planning erosion control will not serve adequately as a grouping
for tax assessment, for example. It will fail one purpose or the
other. Even with an accurate, highly detailed soil survey in hand,
an up-to-date timber cruise may be needed for some kinds of land
classification, or perhaps a detailed map of field patterns and
land use. For still other purposes, additional research to establish
costs for drainage, irrigation, or land clearing is required. To go
ahead and get all these data, along with the detailed soil survey,
on the chance that they may be needed some day, would increase
the cost beyond reason.
Groupings by use capabilities can be made from a good soil
survey with adequate correlative research; but such groupings are
bound to be transitory and will need to be changed with changes
in the agricultural arts, especially in new or undeveloped areas.
Some confusion between soil maps and land classification has
resulted from assumptions of 25 years and more ago that soils
were defined in terms of soil profile alone. (Regrettably, some in
the Soil Survey staff once made this error, too.) Actually, as
already explained, landscapes are classified and mapped in soil
surveys, not simply soil profiles. Some who accepted the early
definition of a soil type as a profile, and who realized that any
mapped area had actually a range of profiles, attempted to get
around the dilliculty by conceiving "land types" or "natural land
types" as mapping units defined in terms of soil profile, slope,
stoniness, depth (including truncation by erosion), and the like.
Such a definition of "land type" is not necessarily different than
the present concept of "soil type." But some went further too, and
included, under the same name, other mapping units now recog-
inized as soil phases or soil complexes. This led to great confusion,
especially in the absence of nomenclature and definitions to differ-
entiate kinds or groupings of "land types." As nearly as one can
make out, these "natural land types" can be placed in soil
classification as (1) soil types, as now defined in terms of all soil
characteristics, including slope, stoniness, and depth, as well as
soil profile; (2) phases of soil types; and (3) associations (or
complexes) of soil series, soil types, or phases. By using defined
units in soil classification, one may go ahead, with orderly
abstraction, to the higher taxonomic groups and to soil associa-
tions for generalized maps.
SOIL SURVEY MANUAL
Clearly it is best to use soil classification and nomenclature
throughout.; Then the results of research and experience can be
utilized at all levels of generalization. It is dillicult to see the need
for the "natural land type." Assuming that "land types" could
be somehow standardized and research results related to them,
they still remain an inadequate basis for genetic classification.
For this, we must fall back on soil classification. Future progress
in taxonomic land classification seems to lie primarily along the
line of improving our soil classification and of including better
definitions of the categories, the individual units within the cate-
gories, and the geographic associations of taxonomic units. There
appears to be no other reliable basis for a scientific classification.
Conceivably, one might develop a nongenetic system, based wholly
upon morphology, but the prospects are dim.
Classification of social units of land.-The very term "land"
itself connotes use. The broad use classes include: (1) Cropping,
(2) grazing, (3) forestry, (4) recreation, (5) mining, (6) urban,
(7) public services (highways, railroads, airports, electric power
lines, cemeteries, and so on), (8) wildlife preservation, and (9)
protection (land managed to protect water supplies or other
lands). Some of these are often combined, as for example, for-
estry, protection, recreation, and wildlife preservation. Besides,
some land is essentially not capable of producing materials or
services of value and may be called wastclafnld. One might add still
another class as idle land-land capable of producing but not now
The soil survey is concerned primarily with the first three use
classes-cropping, grazing, and forestry-but also has a great
deal to contribute to management plans for the others. Some
kinds of soil can be used only in certain of these general use
classes. That is, some are not useful for cropping but may be
used for forestry or grazing. Other kinds of soil can be used in
any of the ways listed, except perhaps for mining. Thus, often
the same kind of soil has a different set of capabilities within
these several broad use classes. Generally, of course, people tend
to use the soils for the most intensive use for which they are
economically capable. But there are many exceptions. Usually soils
unsuitable for farming are used for forestry, recreational parks,
and the like; but in a densely populated community on highly
productive soils, some of those productive for crops may need to
be used for wood lots, parks, and public services.
In the classification of specific tracts of land-farms, ranches,
forests, pastures, or gardens-according to potential productivity,
say for tax assessment, or of prospective tracts according to
irrigability, assumptions of the use class must be made. The
determination of the use class of a particular tract is partly a
matter of the potential productivity of the kind of soil, and partly
a matter of its geographic position and size in relationship to
other kinds of soil, to existing or proposed roads, canals, wells,
and markets, and to other land tracts.
Except for miscellaneous land types as defined later.
32 U. S. DEPT. AGRICULTURE HANDBOOK 18
For example, one cannot assign a soil area to use for crops
unless the area is large enough for an economic unit. Thus, in
regions of soil dominantly suited only to grazing or forestry,
small areas of soils well suited to crops must be assigned to the
other dominant use, except as they may be located strategically
at a ranch or forest headquarters. Soils suitable for grazing
cannot be so used, at least with full intensity, in the absence of
a water supply. On the other hand, a small area of soil suited only
to grazing or forestry, but surrounded by a large area of soil well
suited for crops, may be little more than wasteland if no economi-
cal management plan can be developed for it.
Although distance from market does not directly affect the
classification of taxonomic soil groups or land classes, it may
greatly affect the classification of social land units or tracts. As
a simple example, we might imagine a large area of Chestnut
soils, well suited to the usual range of crops, extending out from
a railway station. For the first 5 or 6 miles potatoes may be
grown in the rotation. At greater distances from the market,
wheat may dominate, first primarily for direct sale and, at
greater distances, with increasing amounts used for stock feed.
Finally, a place is reached where essentially all the crops, both
forage and grain, are marketed through livestock. From an
analysis of production and marketing costs a schedule may be
prepared showing the percentage reduction in the basic rating
of the units because of this distance factor. Then too, the distance
must be corrected according to transport facilities: Poor roads
must count more than good roads. The relationship between the
effective distance and the rating factor is a second order differ-
ential equation, not a linear one, since the differences in costs of
marketing per acre of cropland between, say, 5 and 6 miles are
much greater than those between, say, 35 and 36 miles because
of the difference in use.' Somewhat similar schedules are needed
for land units with intermittent water supply.
The contrast between simple taxonomic land classification and
the classification of specific land tracts may be illustrated in a
system designed for classifying land according to irrigability.
As a first step, a detailed soil classification and map is made for
the area. For purposes of planning the layout of the project, the
soils are grouped according to their arability under irrigation,
without regard to location within the area. Such a soil grouping,
or "land classification," and map predict what would be the result
of irrigation for every part of the area. Then questions need to
be raised about the accessibility of specific tracts of arable soils
to roads and canals and about the combination of various soil
areas into economic farm units. Some areas of soils cannot be
irrigated economically, of course, because of their unresponsive-
ness or likelihood of deterioration, regardless of location; some
other soil areas are highly suitable except where isolated in small
Such an equation and its development is explained in A METHOD OF RURAL
LAND CLASSIFICATION by Charles E. Kellogg and J. K. Ableiter. U. S. Dept.
Agr. Tech. Bul. 4(i9, 30 pp., illus. 19:15.
SOIL SURVEY MANUAL
tracts that cannot be reached economically or fitted into a farm
unit; and areas of other kinds of soil are called irrigable if water
can be supplied conveniently, but nonirrigable if water charges
Thus, two quite different maps of the same area, both accurate,
might be called "land classification according to irrigability." The
first one represents the distribution of taxonomic groups and
might better be called, perhaps, "a grouping of soils according
to arability under irrigation." The second map, made on the basis
of the first one with consideration of the additional factors of
location, is a classification of geographically defined areas and
should be called, perhaps, "a classification of land according to
irrigability." This second map follows an accurate development
of the first one from a detailed soil survey. Besides serving this
immediate purpose of developing the land classification according
to irrigability, the detailed soil survey is used for developing
individual cropping and soil-management systems optimum for
the specific kinds of soil that were grouped into the more general
The classification of specific geographically located areas of
land ordinarily must take account of those characteristics of
place that influence decisions among the land-use classes and the
decisions about relative intensity of use within the classes. In
classifying land for tax assessment, for example, the soil units-
types and phases-are first rated according to their productivity
under alternative systems of management, within each use class,
on a taxonomic basis. Secondly, the use classes of the geographic
land tracts-sections, forties, or farms-are determined. Many
tracts have mixtures of the use classes, say both cropping and
grazing. Thirdly, ratings of the taxonomic groups within the use
classes for each geographic land tract are adjusted according to
distance from market, water supply, and so on, as these influence
This brief discussion has dealt only with a few principles and
examples, but it is hoped that readers may test old schemes of
"land classification" and new ones certain to be proposed. Further
discussion would scarcely be appropriate in this Manual. No
general guides for "land classification" exist, partly because of
the wide variety of activities included by at least someone under
Rural land appraisals for determining the value of land as
mortgage collateral or for tax assessment might be regarded as
special kinds of land classification. Social land units, mainly
farms, are evaluated in terms of potential production within the
institutional and legal environment.
Tax assessment.-Some of the essentials of a method of land
classification for tax assessment have been outlined as an example
under the heading Multiformity of Land Classes (p. 28). For accu-
rate work, a basic detailed soil survey is required, partly because
U. S. DEPT. AGRICULTURE HANDBOOK 18
of the need for indicating the relevant factors in relation to farm
boundaries, and( partly because adjustments will need to be made
from time to time as conditions change.
If the basic soil factors are recorded, as in a basic soil survey,
reinterpretations and regroupings in the light of changed condi-
tions can be made easily and in an orderly fashion. But if they
are not and only judgments of soil productivity, or of soil groups
based on such judgments, are recorded, each revision will require
a complete resurvey. For example, let us think of a modern
detailed soil survey that indicates 150 or so separate kinds of
soil for some area, like a county. These units may be grouped
into 5 or 10 productivity classes, or into any other number of
classes, according to the accuracy required and the availailability
of precise data for evaluating differences in responses to manage-
ment. If, however, only these classes are mapped, the survey is
soon out of date. If the soil types and phases are accurately
mapped, the groupings can be readjusted and revisions made in
the appraisal of specific tracts without additional mapping.
Besides the basic soil resource, the appraisal may need to take
account of farm improvements-buildings, fences, and the like-
according to the State laws governing appraisal. In some States
improvements are not taxed; in most they are. If these improve-
ments are appraised in terms of replacement value, absurd
results may be had, say where previous owners have constructed
buildings far larger and more elaborate than the farm unit
requires. Often the laws require that land must be appraised
according to its productivity in the most intensive possible use,
say for crops, even though it is actually used for extensive
grazing or forestry. In the various States special statutes may
permit present use to carry some weight. Laws vary widely in
the degree to which potential use of farm land for urban or
suburban residences must be weighted in assessment. Presumably
the ideal in assessment is to make appraisals, according to
potential productivity, that differentiate fairly among all the
properties. Everyone realizes that excessively high taxes are
unfair. A great deal of land that has reverted to the State because
of nonpayment of taxes would have remained in private hands had
the assessments been reasonably based upon the productivity in
such uses as forestry and grazing, instead of on a presumed
productivity for farming. But very low taxes are also unfair.
Speculators may be allowed to hold undeveloped or only partially
developed land needed for settlement at little or no cost-land
which they hope to sell or use later at great profit.
Before a proper job of soil groupings and alternative ratings
for the various use groups can be developed, and especially before
attempts are made to appraise social units, a study needs to be
made of both common laws and statute laws that influence
assessment. Then appropriate schedules can be developed and
adjusted ratings of the taxonomic groups made, in terms of the
combinations of present characteristics that need to be dealt
SOIL SURVEY MANUAL
with, for each property within the area. Nearly every area
presents special problems.
Appraisal for loans.-An accurate detailed soil map with ratings
of the individual soil types and phases according to crop poten-
tialities, estimated yields, and long-time effects of the alternative
management systems furnishes the best basis for estimating the
productivity of a farm and its basic long-time value. It is, of
course, helpful to have also records of the individual farm
The appraisal of a farm cannot be based, however, upon the
soil alone. The distance from market and other characteristics of
place must be considered as they affect the kinds of uses for the
farm and the productivity of the farm unit. Buildings, fences,
and other improvements need to be evaluated in relation to the
potential use of the farm unit, as well as water supply, noxious
weeds, and the like.
Besides the basic value of the land and its improvements in
relation to potential use, the loan appraiser can scarcely escape
taking account of the prospective manager of the farm and his
skill in relationship to its potentialities.
SETTLEMENT OF NEW LANDS
For centuries land settlement was on a trial-and-error basis.
Those fortunate enough to find responsive soil in an area large
enough for effective community development, and able to adapt
their practices to kinds of soil new to them, were successful. Many
thousands of settlers were not so fortunate; their work and
efforts care to little or nothing, and their most productive years
Through the use of soil surveys these wastes can be largely
avoided, at least those due to improper soil and lack of advanced
knowledge of what soil management practices to follow. Some
exceptions must be allowed for little known kinds of soils never
before used by civilized man equipped with the tools and services
of modern industry. But the number of these is really small
outside the tropics.
It must be emphasized, however, that the soil survey of a new
or undeveloped area needs to be correlated with soil conditions
in known areas. The necessary predictions of crop adaptability,
yields, and management requirements will need to be based, in
new areas, upon research results and farm experience gained
from similar soils elsewhere, although perhaps not identical ones.
In planning a community, the soil map is useful in locating
roads, schools, and other public services in order to keep costs
at a minimum and provide orderly settlement as compactly as
possible. Helter-skelter settlement with individual settlers far
from one another, even though on responsive soils, raises serious
social problems and results in high costs for medical facilities,
transport, and schools.
In a new area, usually the best procedure is to make a recon-
naissance or schematic soil association map (as defined earlier)
U. S. DEPT. AGRICULTURE HANDBOOK 18
from existing data and scattered observations in order to identify
the most promising places for settlement. This map serves for
broad planning of highways and other public services. Then
detailed soil surveys should follow in the various parts of the
area according to priority of development, considering soil
character and other relevant factors. Beyond these considerations,
the use of the soil survey for settlement is not unlike its use in
GUIDANCE OF PROSPECTIVE FARM BUYERS
The modern soil map and report furnish the prospective farm
purchaser with more relevant information upon which to make
a decision than does any other single publication. This point is
important, and those writing soil survey reports need to bear it
in mind. A part of the use of the soil survey for this purpose
parallels its use for land appraisal for loans, already briefly
outlined. In addition, it gives a picture of the surrounding land
and the potentialities of community development. The soil map
and report help a prospective buyer select the area in which he
wants to buy before he gets down to considering a particular
farm. The report explains the farming systems followed by other
farmers, the crops grown, the market facilities, and so on. In
short, the soil survey report and map should give the prospective
buyer a clear picture of the principal potentialities and problems.
After reading from the soil map the kinds of soil on a farm he
may be considering, the prospective buyer can consult the tables
of yield predictions and management requirements and develop
a tentative farm plan with budget estimates of expenses and
income. For accuracy, these need to be adjusted to other soil
differences due to past management. Where practicable, he should
compare these estimates with other budgets from similar farms
as another check.
No matter how accurate the soil map or complete the supporting
data, purchasers should be advised to visit a farm before making
a final decision. Factors that are important to an individual
family defy accurate description in writing and figures.
Most land users have some sort of plan to guide their opera-
tions. Some farmers have only a simple plan of the crop pattern
for the following year; others have carefully prepared plans in
writing, with a map, for several years in advance-plans that
are revised with the seasons. As science and technology are used
more and more for optimum sustained production, individual farm
planning becomes increasingly important. This kind of planning
is usually called "farm planning," since it deals mainly with
decisions made within farm boundaries.
The term "rural land use planning" on the other hand, is
commonly used for policies and programs that influence the use
of lands in a whole community or area containing many individual
farms or other units of operation. Examples include the planning
of irrigation or drainage districts, rural transport systems,
SOIL SURVEY MANUAL
electric power distribution lines, flood-control structures, large
dams, public land acquisition, rural zoning, and the like.
Many county plans or goals have been made by farm leaders
and agricultural advisers jointly. These vary widely in detail and
scope." For the best development of these plans or programs, a
detailed soil survey and carefully generalized soil association
map are most helpful. Because the soil survey for individual farm
planning needs to be detailed, it is difficult to obtain a view of the
soil resources in the whole community and in the contrasting
parts of counties except with a soil-association map generalized
For many planning purposes, it is helpful to the users to have
the boundaries of soil associations as an overlay on the detailed
soil map and also separately on a smaller scale map showing the
roads, say on a scale of about 1 or 1/! inch equals 1 mile. As has
already been pointed out, the experience gained from pilot-
research farms, demonstration farms, and from the analysis of
other farms can be synthesized by soil associations in order to
guide advisory programs and other public programs designed to
eliminate handicaps for economic sustained production.
The planning of irrigation.-An especially detailed soil map
is required in planning irrigation. This map is often a difficult
one to make because soil characteristics need to be considered
in relation to a very different environment than the natural one.
Deep layers that contain soluble salts or that are impervious to
water, which may have little or no influence on the soil under
desert or semiarid conditions, may be very important to its
behavior under irrigation. Soils that are well drained naturally
may become swamped with extra water. The soil survey must
predict such conditions and whether or not they may be overcome
and, if so, by what methods.
Here, too, generalized maps, based upon the detailed ones, are
needed for planning the transport and water facilities in the
area as a whole and for arriving at a final map of irrigability as
explained previously in the discussion of land classification.
The planning of drainage.-In principle, planning of drainage is
similar to the planning of irrigation. Here also, soil characteristics
of little influence in the natural state become very important
when the soil is drained. Many expensive drainage projects have
failed because the soils were unproductive after drainage. In
some instances, the soils were very sandy, and after a brief period
'A large number of these have been developed. This is not the place to
review this work in detail. A recent example, among a great many, is
the one already cited-MONTGOMERY COUNTY [Alabama] 1FARM PROGRAM (c.
1947). A pioneer rural plan was published by Lee Roy A. Schoenmann as
LAND INVENTORY FOR RURAL PLANNING IN ALGER COUNTY, MICIIGAN (Mich.
Acad. Sci., Arts, and Letters 16: 329-361, illus. 1932) based upon the SOIL
SURVEY OF ALGER COUNTY (U. S. Dept. Agr., 1934). Other examples include:
LAND USE CLASSIFICATION IN MIDLAND COUNTY, MICHIGAN, LAND-USE PLANNING
REPORT (Bur. Agr. Econ., U. S. Dept. Agr. and Mich. State Col. 1940); and
AGRICULTURAL PLANNING, VALLEY COUNTY, MONTANA. (Bur. Agr. Econ., U. S.
Dept. Agr.. and Mont. State Col. 1941. [Processed.]).
U. S. DEPT. AGRICULTURE HANDBOOK 18
of cultivation the organic matter disappeared and the soils
became too loose and too dry in summer for crop growth. Others
had organic soils so acid that enormous quantities of lime were
required for raising the pH to that level necessary for crop
plants. Such additions of lime, besides being costly, often worsen
the problem of other nutrient deficiencies through unbalance.
Drainage of peat lands raises problems requiring the special
attention of soil survey parties. After drainage, organic soils
often shrink and settle unevenly. For this reason tile drainage
frequently fails. The tiles may get out of position. In detailed
soil surveys where drainage of such lands is proposed, through
soundings and study of the deep materials, it is possible to predict
such settling and recommend measures by which difficulties may
be avoided. Often it is necessary to arrange for keeping the water
table nearly constant through combined drainage and subirriga-
tion, even using the same canals.
Public land acquisition.-Land purchase, as for blocking out
National or State forests or grazing districts or for the develop-
ment of public parks, needs to be planned in relation to the use
capabilities of the whole area affected by the purchase units. The
detailed soil survey is an essential basis for appraising individual
parcels, and, if supplemented with a generalized map of soil
associations, for planning the project boundaries. Such purchases
can have pronounced influences on community development and,
with proper planning, can reduce scattered settlement and other-
wise assist the objectives of rural zoning.
The planning of larg-e dams for water storage.-The effects of
alternative locations and heights of the structures upon land use
needs to be taken into account. By carefully plotting alternative
pool lines on the detailed soil map, accurate comparisons can be
made. Thus it may be found that one alternative may cover with
water much more soil productive for crops than another. Then,
after the pool line has been established, plans can be developed
with a detailed soil map for the economic use of all partially
flooded farm units, through reorganization, in order to keep the
"taking line" (the line below which land is purchased) as near
the pool line as possible and thus hold the area of unused land
or public land around the pool to a minimum. In the margins of
some pools, areas of highly productive soil are flooded only occa-
sionally. Such areas may be used for crops to good advantage a
hlrge part of the time if attached to an economic farm unit. With
a detailed soil survey, such planning can be done in an orderly
Planning measures for flood reduction.-Often planning for
flood reduction involves the study of the soil conditions of a whole
watershed in order to estimate infiltration rates, runoff, and the
effects of land management and structures on runoff and erosion.
Costs and benefits of alternative plans should be calculated. A
detailed soil survey, supplemented by a generalized soil-association
map, furnishes a very large part of the basic data for such
SOIL SURVEY MANUAL 39
planning. A full set of predictions and yield estimates under
alternative systems of management for each mapping unit is
essential for accurate results.
Rural zoning.-Ordinances are often developed by county gov-
ernments to promote orderly use of the land." Roads, schools, and
other social services for scattered farms in areas generally
unsuited to farming are very costly for other taxpayers. Such
isolated settlers often increase the fire hazard in forests. By
blocking out areas suited mainly to forestry, grazing, or recrea-
tional use, in which settlement for farming is not permitted,
roads and schools may be provided for the community more
efficiently. Accurate soil maps, along with interpretations made
according to use capability, furnish a sound basis for developing
These few brief examples are only intended to give the reader
an idea of the kinds of use to which soil surveys are often put
in rural land-use planning. All such uses cannot be specifically
anticipated in advance; but when a soil survey is undertaken
in any area, supervisors should be fully aware of any such
possible uses. Even though a rural zoning ordinance does not yet
exist, for example, if it is clearly needed to solve serious problems
of local government management, the soil survey work should be
done in anticipation of its use for that purpose.
ASSESSING POTENTIALITIES FOR SPECIAL CROPS
The economical production, use, and marketing of many special
crops depends upon having more than the minimum volume of
production needed to support canneries, freezing units, or other
special processing and marketing facilities. When a new enterprise
of this sort is undertaken in a community, a large area, often
split among many different farms, must be developed at once,
along with the factory and marketing facilities. Special interpre-
tations of the soil mapping units may be made for the crop, and
interpretive maps prepared from the soil map showing classes
of soils according to their use capabilities for the particular crop.
Such maps serve as a sound basis for assessing the potentialities
for the enterprise in a community and for indicating the par-
ticular farmers that may cooperate.
Foresters are becoming increasingly aware of the importance
of an understanding of soils and their relation to growth, stand
composition, and other factors affecting optimum forest manage-
ment. Even the incidence of certain forest diseases, like little-leaf
of shortleaf pine for example, is related to groups of soils. The
soil survey makes possible the synthesis of results from research
and from experience and the orderly application of the available
SSee the following: ELY, R. T., and TWEHRWEIN, G. S. LAND ECONOMICS.
512 pp., illus. New York. 1940; and WEIIIVtWIN, (. S. THE ADMINISTRATION
OF, RUIUAL ZONING. Jour. Land and Pub. Util. Econ. 19: 264-291. 1943.
U. S. DEPT. AGRICULTURE HANDBOOK 18
knowledge, in forest management in much the same way as in
Soil surveys are being used increasingly in engineering work,
especially in highway and airport planning and construction and
for predicting trafficability of heavy vehicles. The basic facts
about soils needed to predict their behavior in fields include most
of those needed to predict their behavior as subgrades or founda-
tion materials. The several soil properties have different rele-
vancies for the two interpretations-agricultural and engineering
-but the same basic classification serves both.
Detailed soil maps are helpful first of all in planning locations
for structures and for predicting the problems of construction
and maintenance to be dealt with. Especially in the absence of
detailed geological surveys, they are useful in locating such
materials as sand, gravel, clay, and suitable "topsoil" for dressing
banks and other areas to be planted.
For detailed highway and airport planning, a highly detailed
original survey is usually needed on a scale of about 1:1,200,
using the same basic soil classification as that described in this
M1,nanl, with such refinements as may be required, especially for
indicating the physical properties of deep strata. After engineer-
ing tests on soil horizons have been made and classified by soil
type, each type can be characterized and its behavior subsequently
predicted without extensive testing. Classification by tests alone,
unrelated to genetic soil types, gives little that can be used as
a basis for prediction at a new or proposed site without additional
time-consuming and costly testing.
Since the interpretation of soil classification and soil maps for
engineering purposes is a highly specialized field in itself, the
reader is referred to a special manual on the subject,' and to a
summary of soil surveys in the United States as they pertain to
Soil surveys, besides their many widely recognized uses, also
serve a host of others to which some attention must be given.
For many areas they are the most complete base map and are so
used in the absence of up-to-date topographic or planimetric
maps. This fact, and the fact that detailed soil maps, detailed
topographic maps, and detailed geological maps are often used
to supplement one another, emphasizes the need for geodetic
accuracy, standard scales in publication, standard symbols, and
correct naming of features.
Soil maps have been used to locate and design pipelines. They
are helpful in locating radio stations. With interpretation, they
MICHIGAN STATE HIGHWAY DEPARTMENT. FIELD MANUAL OF SOIL ENGI-
NEERING. Rev. ed., 304 pp., illus. Lansing. 1946.
SOLMSTEAD, F. R., HICKS, L. D., and BODMAN, G. B. ENGINEERING USE OF
AGRICULTURAL SOIL MAPS. Highway Res. Bd. Bul. No. 22, 128 pp., illus.
SOIL SURVEY MANUAL 41
can be used as maps of surface geology. They are useful in
studying land form and geomorphological processes. With study
of sample areas, they can be used to construct maps of the
original vegetation and to predict successions of plant cover.
The hazards of nutritional deficiencies among plants and even
among animals may be anticipated from soil maps where the
relationships of deficiencies to soil types have been identified
through correlative research at sample sites. In recent years,
important relationships have been worked out between many
soil types (and soil groups) and deficiencies of such trace elements
as copper, boron, manganese, molybdenum, iron, cobalt, and zinc,
as well as of phosphorus, potassium, calcium, nitrogen, magne-
sium and sulfur. By no means all important soil types have been
characterized, especially for the trace elements, and much more
research is needed. As already explained, recommendations for
an individual field depend partly on previous and current manage-
ment as well as on soil type; yet the area where these deficiencies
are likely, and the general practices to be followed, can be
interpreted from a proper soil map."
With generalized and schematic soil association maps, broadly
defined agricultural potentialities and problems that relate to the
soil or soil use can be seen regionally, nationally, or even on a
world-wide basis of comparison.
Since all places in the world having the same combination of
soil genetic factors have the same kind of soil, knowledge gained
through research and experience in one place is relevant to all
like places. Contrariwise, good practices for sustained production
on one kind of soil may be wasteful or even ruinous on a different
The need for close correlation between those engaged in soil
surveying and other researches is obvious if proper definitions
and predictions are to be developed for soil types and if full and
accurate use is to be made of other research results. This is true
internationally as well as nationally. To make optimum use of
agricultural science in any country, it is essential to have a
consistent world-wide scheme of soil definition and nomenclature.
That is, the results of competently managed research on a well-
defined Latosol, Podzol, or Chernozem are useful in all countries
having soils like the ones investigated, regardless of where the
work is done.
Much work has been done in this field of soil geography. More
is needed. The unrealized opportunities for improving the plan-
ning of agricultural research and for increasing its effectiveness
to all are very great. Fortunately, as this Manual is being pre-
pared, greatly increased emphasis is being given to soil classifica-
tion and mapping in many countries and to the exchange of soil
scientists and of information about soils.
SSee Ignatieff, as cited in the General Bibliography.
.i",. 31 51-
PREPARATION FOR FIELD WORK
Before going to the field for survey work, plans are made and
the essential materials and equipment assembled.
Most soil survey work in the United States is conducted as an
integral part of the soil research programs of the United States
Department of Agriculture and the State agricultural experiment
stations. Besides, other State and Federal research, service, and
educational agencies cooperate in projects of special interest to
them by furnishing personnel or materials.
Many technical details and the services of several kinds of
specialists are involved in a soil survey. Besides the soil scientists
in field and laboratory, at least some assistance, often a great
deal, must be had from geologists, plant scientists, and others.
Skilled photogrammetrists, cartographers, draftsmen, and editors
are essential to the work. Several agencies are usually involved
as participants or as interested users of the results.
A clear understanding of the work to be done and the r81e of
each participant needs to be had at the start. The general
specifications, plan, and assignment of professional workers are
set forth in a Soil Survey Work Plan, drawn up by the supervisory
scientist, with the help of those responsible for cartography and
laboratory services, and agreeable to the sponsoring agencies.
Above all, a qualified scientist needs to be selected for party chief.
Upon him, more than upon any other individual, depends the
thoroughness of the research and the quality of the final soil map
The essential items of the Soil Survey Work Plan are:
1. Nam:e, location, size, and boundaries of survey area. (Include sketch
map for areas other than whole counties.)
2. A paragraph describing the principal physical features of the area.
3. The names of initiating and cooperating agencies.
4. Reasons for the survey, together with any special uses to be made
5. Type of survey (detailed, detailed-reconnaissance, reconnaissance of
soil associations) and features to be mapped, including any special
features not included in the standards for a basic soil survey.
6. Field and publication scales for the maps.'
7. An annotated list of previous surveys of soil, relief, geology, or
8. Equipment and transport needed and agencies responsible for
9. Names of proposed workers (and agency of each) for soil survey
party, including' party chief.
10. Kind. scale, quality, source, and availability of base map materials
and the primary control in the area.'
11. Scale and other features of map to be published and method of
construction from field sheets.'
12. Plans for preparation and publication of report.
44 U. S. DEPT. AGRICULTURE HANDBOOK 18
13. Date for initiating field work; location of first field headquarters,
and estimated date for completion of field work.
14. Plans for supplementary laboratory work and scientists responsible
15. Estimated costs by contributing agencies:
(a) Field mapping by man-days, including salaries, travel, and
(b) Supplemental research and summaries for soil ratings and soil
(c) Supplemental laboratory work.
(d) Map preparation and editing.'
SDeveloped jointly with Cartographic Section.
2Developed jointly with laboratories of cooperating avI nciies.
ASSEMBLY OF CARTOGRAPHIC DATA
The use of good cartographic base material is essential for a
successful soil survey. On it depends the accuracy of plotting the
soil boundaries and symbols, the rate of progress, the methods
and costs of map construction, and the quality of the published
map. Since all these items directly affect the cost and accuracy
of soil maps, supervisory scientists need to give the assembly of
cartographic materials first priority once an area is selected
Even the order in which areas are taken up for soil survey
should be guided by a study and analysis of available cartographic
data. That is, no area should be selected for survey in advance
of aerial photography or equally good base material unless the
most compelling reasons exist for doing so; and areas having
good topographic base maps made with the aid of aerial photo-
graphs should be given preference.
Preliminary study and analysis.-Before its selection for use
in the field, cartographic material needs to be studied in relation
to all operations in both the field and the cartographic office,
considering accuracy, economy, any special needs of a cooperating
agency, and efficiency of use by the field party and by the
cartographers. All available cartographic material is considered.
Some may be helpful even though it is not used directly as the
If new aerial photography is under contract, usually a soil
survey should be postponed until the photographs are released.
The availability of new topographic maps, still in manuscript
form and not yet generally available, should influence the selection
of a specific area. Although uncontrolled aerial mosaics may
appear useful at first glance, in the final analysis they may be
more expensive than individual aerial photographs because of
poor quality, lack of stereoscopic coverage, and inaccuracies.
Topographic maps made with high standards of accuracy may
have to be discarded because of insufficient detail and small scale.
The efficient use of aerial photographs may be limited in some
areas by insufficient control for constructing an accurate base
Without such an analysis, an area may be selected for which
so little good material is yet available that costs for field work
SOIL SURVEY MANUAL
or map preparation, or both, may be very high; or a poor
combination of materials may be selected from among those
available. Such failures in initial planning lead to inaccurate soil
boundaries, excessive costs, and substandard published maps.
Plans for the survey are worked out jointly by soil scientists and
cartographers, so that all costs-for field work, map compilation,
and publication-are taken into account. A minor change in field
operations, for example, may have a large influence on later costs.
Locating material.-So many agencies obtain aerial photog-
raphy, prepare aerial mosaics and planimetric and topographic
maps, and establish control that the field scientists cannot be
expected to know all that is available or about to become avail-
able. Although some agencies release map information periodically,
these reports do not cover many activities in planning and opera-
tional stages. Since most Federal mapping agencies and many
commercial firms maintain offices in the Washington area, the
Cartographic Section of the Division of Soil Survey maintains
liaison with nearly all map-making groups. It is a regular func-
tion of the cartographic office to maintain records of all available
materials and of work in progress and to seek materials from
all agencies for any new survey area. In this way, it is possible
to obtain complete information on the status of aerial photog-
raphy, mapping, and control activities for any area in the
Selection of scale.-Many factors need to be weighed together
to determine the best scale to use for a soil survey.
The purpose of the map needs first consideration. Since most
detailed soil maps are designed to carry the data needed in plan-
ning efficient farming systems, the map must have large enough
scale to indicate areas of significance in farming, either by boun-
daries or by defined symbols. This does not mean that the scale
needs to be large enough so that field boundaries, terraces,
ditches, and farm buildings can be plotted directly on the soil
map. Most farm plans should be drawn on enlarged aerial photo-
graphs or other large sheets so that details important only to the
specific farm may be written on them. Soil maps on such large
scales would be too unwieldly to file and use. The scale of the
soil map needs only to be great enough to permit accurate plotting
and recording of the significant data.
If the survey is reconnaissance-with a generalized or sche-
matic map of soil association and only samples of each association
in detail-the scale can be much smaller.
Generally, the scale of mapping increases with the intricacy
and complexity of the soil pattern and especially with intensity
of soil use or potential use. The patterns of soil types and phases
are very complex in areas of Ground-Water Podzols and Half
Bogs or of Lithosols and Alpine Meadow soils, for example, but
the low potentialities for use argue against the practicality of
highly detailed mapping except in sample areas to define the
mapping complexes or associations. Where small areas of soil
U. S. DEPT. AGRICULTURE HANDBOOK 18
must be enclosed with boundaries, the scale needs to be large
enough to show them without exaggeration and to permit placing
clear symbols in them. If field sheets have a large proportion of
the symbols outside of the areas they represent and keyed into
them with an arrow, the scale is too small, excessive detail is
being mapped, the symbols are too long, or there is some combina-
tion of these evidences of poor planning.
The scale should be no larger than necessary to show the
details required for the objective of the survey. A large increase in
scale increases the number of separate sheets to deal with, the
amount of joining of sheets, and costs for compilation and
The scale of manuscript maps made in the field or generalized
from highly detailed field sheets needs to be reasonably close to
the publication scale. Except in special surveys, where the field
sheets indicate data not to be published, and photographed copies
of them serve the special purpose, the field scale should rarely
be more than twice the publication scale. Otherwise the published
map is likely to be too complex for easy reading or data on the
field sheets must be omitted. The selection of only part of the
data from the field sheets increases compilation costs and the
chances for error. Some poor soil maps have been made at great
cost by publishing at 1 inch to the mile (1:63,360) work done
in the field at 4 inches to the mile (1:15,840). If a field scale of
around 1:7,920 is clearly needed and the map is to be published
at 1:31,680, a manuscript map (besides the field sheets) ordinarily
is required at some scale above 1:15,840.
In the United States most detailed basic soil maps are now
made with field scales between 1:15,000 and 1:20,000 and pub-
lished at 1:24,000 and 1:31,680. Yet, very detailed surveys, say
in irrigated areas or other intensive areas of complex soils, may
be made at field scales as large as 1:5,000. For detailed highway
and airport planning, soil maps are often required at scales as
low as 1:1,000, but those rarely need to be reproduced in large
editions. Scales for soil association maps made in reconnaissance
surveys may run from 1:20,000 to 1:500,000, depending on the
Except for detailed-reconnaissance surveys, uniform scale
should be used throughout an area. Mappers using base material
of varying scale are likely to map the soils in varying detail also.
A lack of uniformity in the kinds and sizes of soil areas shown
greatly reduces the usefulness of the soil map, since it presents
a distorted picture of the soil pattern. Such distortion can be seen
on a few published soil maps for which field sheets of unlike scales
had been assembled to a uniform scale.
Much cartographic base material is flexible enough to permit
reproduction at a number of scales for field use. Many aerial
negatives have a scale of 3.168 inches equal 1 mile (1:20,000).
Prints of excellent quality and detail can be had at scales from
2 inches equal 1 mile (1:31,680) to 8 inches equal 1 mile
(1:7,920). Of course, aerial film may be at various scales; yet
SOIL SURVEY MANUAL
reduction and enlargements are usually satisfactory within
one-half to three times the original scale. Aerial mosaics or
planimetric and topographic maps can be considerably enlarged
or reduced to appropriate scales for field mapping.
The cartographic laboratory of the Division of Soil Survey
is equipped to prepare enlargements and reductions of aerial
photographs, aerial mosaics, planimetric maps, and topographic
maps as may be required in cooperative soil surveys.
Since it is usually possible to obtain the base material at a
proper and uniform scale, it is important to decide on a definite
scale for the soil survey in the planning stage, and to make the
original requests for material at that scale. This is far more
economical than attempting changes in scale after the base
material is received. Such changes may require recopying and
cause avoidable delays.
Factors determining type of material selected.-Frequently
two or more kinds of cartographic material suitable as bases for
soil mapping may be available. An area may be wholly or partly
covered by two or more types of aerial photographs, aerial
mosaics, planimetric maps, or topographic maps. The choice of
materials depends upon their relative advantages for the whole
job, including map compilation and reproduction as well as field
use. The base material selected must be adequate for the whole
job, not for just one activity alone.
Uncontrolled aerial mosaics, for example, may appear advan-
tageous for field use, yet they may be wholly unsatisfactory for
constructing the final map because of inaccuracies. Obsolete or
substandard maps present similar problems. Such maps often
require so many revisions that their value is offset by time-
consuming corrections in both field and office. Work plans calling
for the use of such materials, made without analyses of the whole
process, have led to high costs in relation to the accuracy of the
Available materials of possible use may include aerial photo-
graphs, of single or multiple lens, aerial mosaics of varying
accuracy, photo maps, planimetric maps, or topographic maps.
If no suitable base maps or aerial photographs are available, and
the survey must be made, the field scientist may need to make a
map with the plane table or, in wild heavily wooded country,
with the compass. (See pp. 455 to 463.) Where two or more types
of base material must be used, careful evaluation should be made
to obtain uniformity in accuracy, planimetric detail, and scale.
Other uses of the survey besides publication influence the selec-
tion. If, for example, a detailed classification of land tracts is to
be made, as in irrigation planning or in assessment, a different
base may be better and cheaper than that employed for the usual
basic soil survey. Time is sometimes an important element in
selection. Material readily available may be used even though
better material will be available at some later date. If differences
in quality are great, the survey schedule should be altered
U. S. DEPT. AGRICULTURE HANDBOOK 18
Generally, the best base materials for detailed soil surveys, in
order of preference, are single-lens aerial photographs, controlled
aerial mosaics, transformed multiple-lens aerial photographs,
standard-accuracy topographic quadrangles, standard-accuracy
planimetric maps, and original plane-table maps. It is best of all
to have both good aerial photographs and accurate topographic
Relatively large-scale reconnaissance surveys are best made on
controlled aerial mosaics or standard-accuracy topographic or
planimetric maps. Small-scale reconnaissance surveys are made on
many types of general maps having good accuracy and planimetric
detail or on aerial photo indexes.
Since a complete soil survey is expensive, proper selection of
the base material can have a great influence on efficiency. Fre-
quently mistakes in planning are caused by overemphasizing the
cost of the base material. Where aerial photography is available,
costs for pictures rarely exceed 1 to 2 percent of the cost of the
entire field work. Even original aerial photography would seldom
exceed 5 to 10 percent of the total. Yet the base material fre-
quently means the difference between an excellent soil map and
a poor one. Costs of base material need to be weighed against
its use in all operations-field mapping, map preparation, and
publication. The use of low-cost materials may give apparent
savings for the field sheets, but result in doubling the costs of
map preparation and reproduction. Since conditions vary widely
from place to place, no hard-and-fast rule can be given for
selecting base material. Each area must be studied as an indi-
Procedure for obtaining base material.-After the work plan
has been developed and the base material decided upon, it is
furnished through Cartographic Section or, by arrangement with
them, directly from other sources. Plans should be made as far
in advance as possible, since many agencies have small staffs
available for supplying photographic prints and other materials
and some delays are inevitable.
KINDS OF BASE MATERIALS
The characteristics, advantages, and disadvantages of the
principal kinds of base material used in soil mapping are outlined
in the following paragraphs.
Aerial photographs.-Nearly all detailed soil mapping is now
done on aerial photographs. Improvements in them and in their
use and interpretation are being made continually.
Types.-Oblique and vertical pictures may be regarded as two
basic types of aerial photography. Multiple-lens photography is a
combination of the two. Single-lens vertical photographs are best
for soil mapping, although oblique and multiple-lens photographs
can be used. Thus emphasis in this Manual is given to single-lens
vertical photographs flown to the specifications of the United
States Department of Agriculture.
SOIL SURVEY MANUAL 49
Stereoscopic and alternate coverage.-Specifications of the
United States Department of Agriculture for aerial photography
require the overlap in line of flight to be about 60 percent;
whereas the overlap between adjacent flight lines averages around
30 percent. This overlap, with which all ground images appear
on two or more photographs, permits stereoscopic vision of any
ground object within the area. Such photography is said to have
stereoscopic courage; and adjoining photographs are called
If every other photograph in a continuous stereoscopic series is
removed, the remaining series is called alternate coverage, and
adjoining photographs, alternate pairs. Alternate pairs of photo-
graphs overlap only about 20 percent-too little to permit stereo-
scopic study of the entire area. Such alternate coverage is inade-
quate for constructing" base maps by photogrammetric methods
based upon stereoscopic coverage.
Contacts and cnargemnct.-Aerial photography is exposed on
film or glass negatives at a predetermined scale and fixed negative
size. The scale of the photograph depends on the height of the
aircraft and the focal length of the camera. The size of the
negative varies with the aerial camera.
The scale of aerial photography depends on the purpose of the
photographs. Most of the aerial photography for the United States
Department of Agriculture is flown with an 8.25-inch focal length
aerial camera at altitudes of about 15,000 feet. The resulting scale
is approximately 3.168 inches equal 1 mile, or 1:20,000. Such
negatives give satisfactory reductions and enlargements within
a scale range of about 1:7,500 to 1:32,000. Most needs for soil
mapping can be met within this range of scale.
Aerial photographs made directly from the original negatives
are called contact prints. These have the same scale as the nega-
tives. In contact printing no rectification of errors or scale changes
can be made, although poorly exposed negatives can be improved.
Contact prints are the most economical to make. When properly
processed they are best in quality.
Aerial photographs may be readily enlarged or reduced; this
is one of their great advantages as a base for soil mapping. The
process requires projection of light through the negative and
precise adjustments for scale. It is therefore slower and more
expensive than contact printing. Some detail is lost in the prepara-
tion of enlargements, but with skillful operators using modern
processing equipment and the original negatives the loss is
Enlarging has certain advantages. With adequate ground con-
trol, all prints in an area can be brought to a nearly uniform
scale. Prints having excess tilt, causing displacement of objects
and scale changes, can be rectified to minimize the errors. Pictures
for areas having photography at two or more contact scales can
be brought to a common scale. Such operations require more time
than simple enlarging; and for scale-ratioing or rectification,
adequate ground control is essential. Nonetheless, later savings
50 U. S. DEPT. AGRICULTURE HANDBOOK 18
may more than offset the cost of bringing pictures to a common
Satisfactory enlargements from average film should not be
expected at scales requiring more than a 21,-diameter enlarge-
ment from the contact negative. The photograph becomes grainy
and much detail is lost.
Photographs flown for the United States Department of Agri-
culture are usually made with aerial cameras having a negative
size of either 7 by 9 inches or 9 by 9 inches. Enlargements, of
course, increase the size of the photograph as well as the scale.
The following shows how the size of sheets, in inches, varies with
Conftact prints Etn largncmr )ti (nl(trfj m nt
(Srale I:20.000 (Scale I :/;.. (Srcle 1 : .9
3.1,sn in. I mile) 4.00 in. n- I mile) 8.00 i". =- I mile)
Inches Inchc8 Inches
7 by 9 ................ 11 by 14 22 by 27
9 by 9 .............. 14 by 1. 27 by 28
Photo indcxes.-Photographic indexes are available for most
of the photography available in the United States Department of
Agriculture, and in other government agencies as well. These
are prepared by fastening the individual photographs of an area
together. The images are matched and the photographs over-
lapped so that all marginal data are visible. The assembly is then
photographed at a smaller scale, often in several sheets for con-
venient handling. Most indexes available in the United States
Department of Agriculture are on sheets about 20 inches by 24
inches and have a scale of around 1 inch to the mile (1:63,360).
Four to five index sheets cover an average county.
Photo indexes are useful for determining the number and
location of individual photographs within an area. Since the low
cost of the indexes is easily made up in the time saved, they
should always be obtained when available. They are also useful
for schematic mapping.
Advantages and di.sadmvatages.-The greatest single advantage
of aerial photography in soil surveying is the wealth of ground
detail shown. Physical and cultural features that it would be
impractical to show on base maps are represented in infinite
detail on the aerial photograph. Field boundaries, isolated trees,
small clumps of bushes, rock outcrops, buildings, and plant cover
all assist the soil scientist in orientation and in plotting his data.
Photographs increase both the speed and accuracy of his work.
Streams, lakes, and swamps that are difficult to plot accurately
by ground methods become control on the photographs.
Because large areas can be photographed rapidly, field scientists
may be supplied with highly detailed base material in a short
time. Compared to other methods of obtaining original bases with
comparable detail, aerial photography is by far the most rapid
and economical method. Isolated areas, difficult to map by ground
methods, are no handicap to the photographic aircraft, provided
suitable landing fields are within operating distances. Aerial
pictures are especially helpful to the soil scientist faced with the
SOIL SURVEY MANUAL
problem of making accurate soil surveys in wild areas proposed
for agricultural development.
Stereoscopic vision, or the ability to sec depth, is another
advantage of aerial photographs in soil mapping. With photo-
graphs having overlap adequate to permit stereoscopic study, the
soil scientist has before him a relief model of the area, complete
with all of its intricate cultural and physical detail. Such a model
affords an opportunity for study of the area in advance of field
work. His traverses can be laid out most effectively. The study
of plant cover, relief, drainage patterns, and other details helps
greatly in planning the field work. Streams, swamps, and other
features may be tentatively drawn in advance.
Adequate base maps having the necessary detail to carry the
soil survey data can be constructed economically and within a
reasonable time from aerial photographs, provided that the
photography is of good quality, that the ground control is ade-
quate, that modern photogrammetric facilities are available, and
that qualified photogrammetrists supervise the work.
Despite these advantages, aerial photography has some disad-
vantages and limitations in soil surveying. Photographs are
inferior to good topographic or planimetric maps in the following
ways: (1) Elevations are not shown; (2) the photographs lack a
precisely uniform scale throughout the area because of variations
in ground elevations and altitudes of the photographic aircraft;
(3) the soil scientist is forced to handle more sheets than when
using large maps, resulting in more matching, joining, and filing;
(4) differences of scale between adjoining photographs create
some minor difficulties in matching and transferring soil boun-
daries from one photograph to another; (5) distances and direc-
tions cannot be so accurately measured because of distortions due
to tilt, displacement, and other inherent errors; and (6) although
far more detail is shown than on standard maps, it is not always
so legible and more skill is required to interpret it. Many details
on aerial photographs, such as field boundaries, fence rows,
wooded areas, and crops are ephemeral and change more rapidly
than the selected features shown on a standard map. For this
reason old photographs may be more difficult to use than good
maps made from them about the same late as they were taken.
Yet these limitations are small in relation to the advantages.
Procedure for obtainifg.-Approximately 90 percent of the
United States has been photographed during the past 15 years.
The major portion of this photography is suitable for soil map-
ping. Much of it is old and difficult to use because of changes in
vegetation and cultural detail. Areas are being continually re-
flown, however, as changes justify.
Once the survey area is selected the order for photographs
should be placed as soon as possible. Such requests should give
the exact boundaries of the proposed survey, the scale of photog-
raphy needed, whether stereoscopic or alternate coverage is to be
used, and the date the survey is to commence. Any special
requirements, such as weight of paper or finish, should be added
U. S. DEPT. AGRICULTURE HANDBOOK 18
also. Aerial film held by other Federal agencies is normally avail-
able on loan to the Cartographic Section for the preparation of
reproductions. Because of limited facilities, however, it is neces-
sary to have some photographic prints prepared by the agency
having the original film. As prints from much of the aerial film
are in great demand, it often takes a long time to get prints
In estimating the time required to obtain original aerial photog-
raphy, time must be allowed for preparing specifications, award-
ing contracts, photographing the area, and inspection and ac-
ceptance of the work. Perhaps the most uncertain factor is
weather. The frequency of suitable days for photographic flying
varies in different parts of this country and in different seasons.
In places aerial photography taken at some seasons is better than
that taken in others. For example, the best photography in the
southeastern part of the United States is had during the winter
months, when the vegetation least obscures the ground.
Costs.-The cost of original aerial photography varies greatly,
depending on the local weather conditions, availability of airfields,
and so on. That flown to specifications of the United States De-
partment of Agriculture has varied considerably in different con-
tracts from year to year. During the 10-year period 1939-49,
yearly average costs varied from $1.93 per square mile in 1939 to
$4.06 in 1945. Costs in 1949 average $2.71 per square mile, or less
than one-half cent per acre.
Reproductions from original film are furnished from other
agencies at rates based on costs of labor and materials. Within the
United States Department of Agriculture unit costs in 1950 for
reproduction were as follows:
Conltart prints Enlargfement,
(1. 0.".00) ( : 1 .! ;,)
Quan t ity: Ea h Each
1 to 5 ................ ................. $0.80 $1.55
6 to 100 .......... ......... .50 1.00
Over 100 ... ............. .45 .90
County coverage .. .40 .80
Where original aerial photography is available the cost of con-
tact prints in stereoscopic coverage for a county is about 26 cents
per square mile. For stereoscopic coverage with 1:15,840 enlarge-
ments the cost per square mile is 52 cents. These costs are a minor
fraction of the total for a basic soil survey.
Aerial mosaics.-Aerial mosiacs are made by assembling and
matching individual aerial photographs to form a continuous
photographic image of an area. A few photographs may be used
to cover a small area, or hundreds of them may be assembled for
a large one. Several methods of assembly may be used, and the
results vary widely in accuracy and usefulness.
Types.-The two general types of aerial mosaics are the uncon-
trolled and the controlled. The uncontrolled mosaic is made simply
by matching like images on adjoining photographs without the
use of ground control. No corrections are made for scale, tilt, or
SOIL SURVEY MANUAL 53
displacement. Since the photographs are matched by picture im-
ages only, without geographic control of their position, an un-
controlled mosaic is not suitable for accurate mapping and is dif-
ficult to use in map construction. In making a controlled mosaic,
the photographs are adjusted to ground control; distances and
directions are measurable; and the individual photographs are
brought to correct scale and corrected for tilt and displacement.
Each photograph is matched and adjusted so that image points
on the photograph fall in their true geographic positions on the
map grid. Since a controlled mosaic closely approaches the accu-
racy of a good planimetric map, the soil scientist can use it as a
base in soil surveying.
Between the inaccurate, uncontrolled mosaic, on the one hand,
and the accurate, controlled one, on the other, are a wide variety
of semicontrolled mosaics for which different forms of ground
control are used. Thus mosaics vary greatly and must be carefully
checked for adequacy before use in detailed soil mapping.
Advantages and disadvantages.-An aerial mosaic has the ad-
vantage of covering a large area in one photograph. Thus fewer
sheets need be matched. Mosaics can be made to cover a specific
area, like a township, a small watershed, or a drainage basin.
Where controlled mosaics are available, their accuracy over that
of the individual photographs is also an advantage to the soil
scientist in plotting soil boundaries and in transferring them to
In reproducing sheets for field use from a mosaic, a small mar-
gin of overlap can be retained, or the sheets can be reproduced to
match without overlap. This is an advantage, since the soil sci-
entist frequently has difficulty in matching adjoining aerial
photographs that have wide margins of overlap.
A major disadvantage of aerial mosaics in soil surveying, as
compared to overlapping photographs, is that mosaics themselves
cannot be used for stereoscopic study of the area. The great value
of such advance study of an area has already been emphasized.
As with planimetric and topographic maps, the accuracy of
mosaics cannot always be assessed by their appearance. They
must be field checked. Extreme difficulty may be had in the field
with a mosaic that appears in the office to be of top quality. Even
though an uncontrolled or semicontrolled mosaic may be usable in
the field, it may be impossible to construct an accurate map for
publication except at great additional expense. Thus the whole job
should be considered when planning the use of an aerial mosaic.
Preparation.-The Cartographic Section of the Division of Soil
Survey is equipped to prepare a limited number of controlled
aerial mosaics suitable lor soil mapping in areas with adequate
ground control already established. The normal procedure is as
follows: Obtain all ground control in the area, plot it, lay out the
projection, and construct a radial Iplot; obtain the original aerial
film; restitute all prints to fit the controlled grid, using care in
the processing to insure a uniform tone and quality; trim the
U. S. DEPT. AGRICULTURE HANDBOOK 18
photographs, apply the adhesive, and adjust and assemble the
prints on the mosaic board; prepare the necessary sheet borders,
titles, and footnotes; make copy negatives of the complete mosaic;
and reproduce the number of copies required.
Procedure for obtaining.-Where it is best to use controlled
mosaics for a soil survey, the request for such work should be
made well in advance. Facilities are not available for preparing
mosaics for all soil surveys, nor should mosaics be recommended
unless they will expedite field work, use of field sheets by co-
operating agencies, and map publication.
Costs.-The cost of aerial mosaics for a soil survey is naturally
higher than for the individual pictures used; yet in some areas the
use of a good controlled mosaic may reduce the total cost of the
survey. Part of the costs may be charged to the normal cost of the
map preparation. Obtaining control data, preparing the control
plot, and making the necessary sheet layouts are a normal part of
the map preparation in many areas. The aerial prints have to be
supplied to the field party anyway; therefore only the operation:;
of assembling, adjusting, and reproducing the mosaic are added
The cost of mosaics is generally less than that of preparing a
planimetric map for the soil survey and higher than that for
individual aerial photographs.
Photomaps.-The photomap is a form of aerial mosaic. Unlike
the conventional mosaic, physical and cultural features are shown
as they are on a planimetric map, and the sheets are laid out uni-
formly on a definite projection, as is done with standard topo-
graphic or planimetric maps. Photomaps are usually reproduced in
large quantities by offset lithography or some similar process.
Frequently, the planimetry is shown in color. Color emphasizes
and gives greater legibility to planimetric detail, for it contrasts
with the black-and-white photographic background.
Types.-No fixed standards have been established for photo-
maps. Although good accuracy may be generally assumed, because
of the expense of constructing and reproducing a photomap, the
soil scientist should test the accuracy of a photomap before using
it in the field.
Photomaps are usually published in sheets in minutes of lati-
tude and longitude depending on the scale; but in sectionized parts
of the United States, the sheets may be laid out to cover one or
Photomaps vary widely, depending on scale and purpose. Some
are published with only grid lines and appropriate titles and foot-
notes; others show the usual planimetric features-roads, drain-
age, buildings, railroads, power lines, and the like-sharply de-
fined with appropriate standard symbols, and with place names
for the prominent features. On printed copies of some photomaps
the planimetric detail is indicated by overprints in color-drain-
age in blue, cultural features in black, and special features in
SOIL SURVEY MANUAL
other appropriate colors. Such photomaps are sometimes called
'laniisaics. A few photomaps include topography or terrain form
lines, with the contours or form lines printed in brown, as on
standard topographic quadrangles. These are called toposaics.
Photomaps with contours to show exact topography can usually
be assumed to be well constructed and accurate. Where approxi-
mate form lines of the terrain are shown instead, the photomap
is probably made to less precise standards.
Three general types of photomaps can be roughly defined as
follows: (1) Those reproduced in small editions by photography
rather than lithography, in which the photographic background
appears like it would in the aerial photograph, with lines and
symbols in black or white lines on this background; (2) those
printed in large editions by offset lithography in black and white,
with planimetric line work overrun in black on a photographic
halftone background made with a fine dot screen; and (3) those
reproduced by offset lithography in two or more colors, with the
photographic background shown by halftone screens in black or
grey and the planimetry, contours, or other special features over-
run in appropriate contrasting colors.
Advantagcs and disadrnaitolcs.-Since the photomap is an ad-
vanced stage of the aerial mosaic, it has many of the same charac-
teristics, advantages, and disadvantages. The delineation of cul-
tural and drainage features is a major advantage over the con-
ventional controlled mosaic, since it eliminates or reduces the
possibility of errors in interpretation of planimetric detail and the
resulting errors in soil boundaries that may occur when mosaics
are used. The soil surveyor normally spends less time classifying
and delineating such detail with a recent photomap than with an
Normally, the photomap can be relied upon to be more precise
than the conventional mosaic. Photomaps sufficiently precise to
meet the standards for published soil maps can be used readily
in the map assembly.
The disadvantages of the photomap are similar to those of the
controlled mosaic. Photomaps cannot be used for stereoscopic
study of an area. This is a handicap, but not so great a one as that
encountered with the conventional mosaic, on which physical and
cultural details are not delineated.
Because of methods used in producing them in large numbers,
photomaps frequently lack the photographic detail found on
photographic copies of a mosaic. Unless exceptionally fine screens
are used in the offset lithography, the photographic detail repro-
duced is not of high quality.
Procedure for obtain ing.-Photomaps prepared by other or-
ganizations, like other base materials, are obtained through the
Cartographic Section. Full information on the accuracy of photo-
maps needs to be had before their use is recommended. It is not
practical to prepare photomaps for use in soil surveying alone.
U. S. DEPT. AGRICULTURE HANDBOOK 18
Accurate identification of drainage and cultural features require
From time to time, however, photomaps may be produced for
publishing a soil map. A controlled mosaic can he constructed in
advance of field mapping and used by the soil scientist as a base
for mapping. The surveyor classifies the cultural and drainage
features on the mosaic while making the soil survey. By using the
original mosaic as a base, and preparing color separations for
drainage, culture, and soils, the Cartographic Section can prepare
the soil map as a photomap.
Cost.-A photomap costs more than a conventional controlled
mosaic, because costs for field editing, drafting the cultural and
drainage features, and making lithographic reproductions must
be added. The cost of a soil map as a photomap should be com-
parable to that for the conventional soil map, provided color tints
for soils were omitted from the photomap.
Topographic maps of standard accuracy.-A topographic map
presents both horizontal and vertical positions of the physical
features of a land area on a flat plane at definite scales. Published
maps usually show such cultural features as roads, railroads, and
buildings in black; drainage features in blue; and contour lines in
brown. Some also show additional features, such as vegetation,
in overprints of green or other colors.
Most topographic maps published by the United States Geolog-
ical Survey and other Federal maps meeting these requirements
carry marginal notes indicating compliance with the National
standards of map accuracy. The standards for horizontal accuracy
of maps published at scales larger than 1:20,000 prescribe that
not more than 10 percent of the tested points shall be in error by
more than one-thirtieth of an inch. On maps published at scales
smaller than 1:20,000, the error shall not be more than one-
fiftieth of an inch. These limits of accuracy apply only to posi-
tions of well-defined points, like roads, monuments, large struc-
tures, and railroads, which are readily visible and which can be
plotted at the scale of the map within one-hundredth of an inch.
Standards for vertical accuracy require that not more than 10 per-
cent of the tested elevations be in error by more than one-half of
the contour interval.
'Types.-Because of the prescribed standards of accuracy, topo-
graphic maps vary little, even though published by different
agencies. Some differences may be noted in format, scales, boun-
daries of latitude and longitude, and classification and presenta-
tion of planimetric detail-differences due primarily to needs for
meeting specific requirements.
Standard topographic maps are published in quadrangles
bounded by parallels of latitude and meridians of longitude. Gen-
erally, topographic quadrangles are 30 minutes, 15 minutes, 71/2
minutes, or 3:-% minutes of latitude and longitude. Scales vary
with topography and contour interval. The most usual publication
scales are 1:24,000, 1:31,680, 1:48,000, 1:62,500, and 1:63,360.
SOIL SURVEY MANUAL 57
Maps of smaller scale are useful to the soil scientist only for re-
connaissance mapping. Few topographic maps are published at
scales larger than 1:24,000.
Advantages a(d disadvantages.-The reliable accuracy of
standard topographic maps gives them definite advantages in
measuring distances and directions. The topographic pattern is
very helpful to an understanding of soils and in the study of
drainage, irrigation, and erosion cycles. The planimetric detail on
the maps relieves the soil surveyor of a part of this task when
As a base for detailed soil mapping, the topographic quadrangle
lacks the ground detail-field boundaries, isolated trees and
bushes, fences, and similar features-that are shown on a good
aerial photograph or mosaic. The small scale of many topographic
quadrangles and the lack of coverage for large areas are further
disadvantages. Drainage patterns on the standard topographic
quadrangle are not shown in the detail needed for soil maps. Some
old topographic maps are not accurate and need a great many re-
visions. The topographic maps of recent years, made from aerial
photographs, are much more accurate.
In planning the use of topographic quadrangles in the prepara-
tion of soil maps for publication it must be recalled that a great
deal more is involved than simply transferring soil boundaries to
the quadrangles. Their use may or may not reduce costs, depend-
ing on the project.
Where recent, large-scale topographic quadrangles cover all, or
a large part, of a soil survey area they are very useful in publish-
ing the soil map. The use of such accurate quadrangles eliminates
the necessity of constructing a base, which is especially helpful in
areas with much culture. Then too, in densely wooded areas an
accurate topographic map shows more points for location than an
air photo. Such quadrangles serve only as a manuscript base,
however, even after they are assembled into sheets of the size
needed for the soil map. It is still necessary to transfer the soil
data to this manuscript and prepare glass negatives or nonphoto-
graphic metal-mounted blue-line manuscript maps' for the color
separations that show culture, soils, and drainage. These color
separations are then drafted or engraved, new lettering layouts
are prepared, and the printed lettering is applied to the color sep-
arations. Plates for the soil separation tints are then made and
the various tints blocked out. Composite proofs are prepared and
edited. The complete color separations are then copied, litho-
graphic plates made, and the lithographic copies printed.
With old topographic quadrangles, made to less precise
standards and requiring much revision, it may cost more to pre-
pare the soil map than to make a new base from aerial photo-
graphs. The difficulties of transferring soil boundaries and sym-
bols from the aerial photographs and adjusting them to fit old
'See Notes on Map Compilhtion and Reproduction, Appendix III.
i",5 3n 51 5
58 U. S. DEPT. AGRICULTURE HANDBOOK 18
quadrangles, and of revising the planimetry, more than offset
the saving made by using them.
Only recent large-scale topographic quadrangles covering most
of the survey area are recommended for use as a base. It is best
of all if both aerial photographs and recent topographic maps
made from aerial photographs are available.
Procedure for obtaining.-Standard topographic maps are pub-
lished mainly by the Topographic Branch of the United States
Geological Survey, the United States Coast and Geodetic Survey,
and the Army Map Service of the Corps of Engineers.
The Cartographic Section of the Division of Soil Survey re-
ceives new lists and new topographic quadrangles as they are pub-
lished and can supply available topographic maps needed for soil
surveys. In addition, the Cartographic Section can supply infor-
mation about areas in progress, expected dates of completion, and
details concerning the topographic mapping program. Frequently
preliminary proofs or copies of manuscript material may be ob-
tained in advance of publication, where the need is urgent. When
aerial photographs are supplied for a soil survey the Cartographic
Section normally forwards all available standard topographic
quadrangles as well, since such maps are helpful as reference for
place names and for soil study, even though not used as the
Costs.-The topographic quadrangle of standard accuracy is
expensive to construct and publish. It serves many useful pur-
poses, besides serving as a planimetric base for soil maps.
Planimetric maps of standard accuracy.-A planimetric map
presents the horizontal position of the physical features of an
area on a flat plane at definite scales. Unlike the topographic
maps, no vertical distances are indicated. Otherwise, they are
usually published in a form like topographic maps. Although no
generally accepted precise standards for planimetric maps have
been established, many mapping agencies have established
standards that approach or equal those for topographic maps.
Only such accurate planimetric maps are used for soil mapping.
Types.-Although standards for planimetric maps vary more
than those for topographic maps, they are usually published in
quadrangles similar to topographic maps and at approximately the
same scales. Some differences result from variations in the map
needs of the agencies preparing them. As a base for soil mapping,
these differences are minor, compared to accuracy.
Advantages and disadvantages.-Planimetric maps have some
of the same advantages of topographic maps as a base for soil
surveying. A major exception is the omission of topography so
valuable for soil study and interpretation. Then too, accuracy is
less certain. Where accuracy is equal to that of good topographic
maps, planimetric maps are helpful, even though the soil mapping
is done on aerial photographs.
SOIL SURVEY MANUAL 59
Procedure for obtaiing.--As with standard topographic maps,
the Cartographic Section receives copies of published planimetric
quadrangles and can obtain them when needed. Where available,
they are normally supplied with the aerial photographs for
Cost.-Although cheaper than topographic maps, accurate
planimetric maps cost more than the conventional controlled
mosaics for comparable areas.
Other types of maps.-Many other types of maps are published
by public and private agencies. These range from the small-scale
road maps distributed by oil companies to the large-scale detailed
maps used in city planning. Most of these are designed, con-
structed, and reproduced to meet a special purpose. Certain de-
tails on such maps are usually emphasized to meet special require-
ments by exaggerating certain items and subordinating others.
The small-scale road map is a typical example. On such a map the
highways, highway numbers, towns, cities, points of interest, and
mileage distances are prominently shown, while drainage, rail-
roads, pipelines, power lines, and public land lines are omitted
Aeronautical charts are special-purpose maps designed and
constructed specifically for air navigation. The scale is small so
that large areas may be shown on a single sheet. Ground features
prominent from the air are emphasized in bold and simple sym-
bols. Other features of equal importance on the ground but less
noticeable from the air are subdued or omitted entirely. Eleva-
tions are shown in gradient tints, permitting the navigator to
determine quickly the necessary flight altitude over a given area.
Navigation data are shown in bright overprints.
The plats prepared from public land surveys are another form
of special-purpose map, designed to present the data of the survey.
The scale is large, and plats usually include a survey unit, such as
a township. Courses and distances, subdivisions of sections, acre-
age figures, and other data from the survey are shown. Cultural
and drainage features are reduced to a minimum and are accurate
only on the survey lines.
Special-purpose maps of the kinds described in preceding para-
graphs have little or no value as bases for detailed soil surveys.
Such maps are very useful, however, for reference.
For broad reconnaissance soil surveys special maps may be
useful as bases. Aeronautical charts, for example, are useful for
rapid small-scale surveys of large areas. They have sufficient de-
tail for orientation, accuracy is good at the small scale, and the
generalized relief facilitates soil mapping.
The Cartographic Section can supply field parties with many
special maps, including aeronautical charts, geologic maps, forest
maps, coast and harbor charts, conservation survey maps, Census
Bureau maps, Post Office maps, and highway maps.
60 U. S. DEPT. AGRICULTURE HANDBOOK 18
Requirements for equipment vary so widely from area to area
that only those of general use are discussed here. Other sections
of the Man al mention items for special needs. Plane tables, and
accessories for them, and compasses are dealt with in the
SPECIAL EQUIPMENT FOR AERIAL PHOTOGRAPHS
The materials used to delineate culture, soil boundaries, and
symbols on aerial photographs and mosaics should be selected
for ease of use, including correction, neatness, clarity, and per-
manence. These materials are now sufficiently standard that they
may be readily obtained from commercial suppliers.
Pencils.-Despite a wide range in personal preference for types
and hardness of pencils, the basic requirements are the same. The
pencil marks need to be sharp, clear, and legible, but made with-
out scratching or cutting the photographic emulsion. Pencils
should be soft enough to leave a legible line yet not soft enough
to smear with ordinary handling of the pictures. Too soft a pencil
leaves a coarse heavy line that smears a dirty residue over the
surface of the photograph and conceals other data. A very hard
pencil scratches or indents the photographic emulsion, makes ink-
ing difficult, and requires such hard erasures for making correc-
tions that the photographic emulsion may be broken.
Variations in the surface of the aerial photograph and in
atmospheric conditions partly govern the choice of pencils. On
hard and glossy photographs it is necessary to use a soft pencil
for the line work to adhere. On the softer matte finishes, a harder
pencil is better. A softer pencil is used during damp weather or in
humid climates than is used during dry weather or in the desert.
Depending on conditions just mentioned, standard drafting
pencils of good quality ranging from H to 4H are used in dry
regions, and HB to 3B for moist regions or during periods of
Inks.-Inks should be bright-colored, of opaque density, free-
flowing, waterproof, rapid-drying, and of the kinds that photo-
Where the inking is done directly on the aerial photograph, a
standard waterproof drafting ink should be used. If the inking is
done on an acetate or plastic overlay, rather than directly on the
aerial photography, special inks are used that adhere well to these
media and that are easy to handle. These are called acetone inks
and they etch the plastic slightly. Such special inks are too diffi-
cult to remove for use directly on photographs; in their removal
the photographic emulsion is frequently damaged and brown
stains are left. Standard waterproof drafting inks may be used
on acetate or plastic overlays if sealer coatings are applied over
the ink work immediately to prevent rubbing off or chipping.
Such plastic sealer coatings can be applied with a soft cloth, brush,
or spray. They dry rapidly, are transparent, and can be marked
SOIL SURVEY MANUAL
on with pencil or ink. Only a light coat of sealer should be applied.
In preparing some kinds of contact prints from the original
sheets, the sealer sticks to the rollers if too much has been used.
Only those colored inks are used that permit good photographic
copies of the field sheets. Many colors do not photograph well un-
less copied through filters. Such filters may bring out any one
color by subduing another. The use of filters also increases the
time required for copying. Generally, black, red, and brown
photograph well. The photographic qualities of blue, green, and
yellow are normally poor but may be increased by mixing small
amounts of the more photographic colors with them.
Mixed colors, as for example, black mixed with blue, should
contrast on the field sheets and yet permit satisfactory copying.
If a line is to appear in blue on a field sheet yet contain enough
black to permit good photo copies, only a little black ink should
be mixed in the blue.
Transparent overlays.-A number of transparent materials
suitable for overlays on aerial photographs or mosaics are on the
market. These fall into two general classes: (1) Plastics and
(2) acetates. Both can be obtained in a variety of thicknesses
Overlay materials should have dimensional stability. If one
without dimensional stability is selected, difficulties are had in
maintaining registry between the overlay and the photograph and
in matching one overlay with another.
The most useful materials for overlays range from 0.005 to
0.080 inch in thickness. Enough thickness is needed to give stiff-
ness and to avoid curling, yet not so much that sheets are bulky
and difficult to handle.
The best overlays are transparent, with a grained surface.
These have maximum transparency and their surface is suitable
for both pencil and ink work. India ink can be used on ungrained
acetate or plastic if soiling with perspiration or other oily sub-
stances is avoided. A sealer needs to be applied immediately
A transparent dimensionally stable material, with matte finish
on one side and about 0.008 inch thick, is entirely satisfactory.
Types of photo paper.-In ordering aerial photographs and
mosaics it is sometimes helpful for the soil scientist to specify the
type and finish of paper on which the photographs are to be
printed. Of the many kinds, some are satisfactory and others
unsatisfactory for soil mapping.
Most photographic papers are available in three thicknesses or
weights, as light, single, and double. Lightweight papers are too
thin and flexible for most soil mapping. They tend to curl and
lack dimensional stability. Where copies of field photographs thin
enough to use over a light table in transferring data from one
sheet to another are wanted, the lightweight papers are satisfac-
tory. An extra thin paper is made that is especially good for this
62 U. S. DEPT. AGRICULTURE HANDBOOK 18
Single-weight paper is somewhat thicker than the lightweight
papers and is commonly used for printing aerial photographs to
be used only in offices. Even this weight is too light fur satisfac-
tory field use where photographs are handled a great deal and
exposed to variable weather conditions.
Double-weight paper is approximately twice the thickness of
single-weight. It is stiff and does not curl, has a reasonable de-
gree of dimensional stability, and is best for photographs that
are to be used in the field for soil mapping.
Photographic finishes are classed broadly as glossy, semimatte,
and matte. The surface of a glossy photograph is too slick and
polished to accept pencil or ink well, and cannot be used con-
veniently. Semimatte and matte finishes take pencil and ink well,
and these finishes are used on photographs on which survey data
are to be plotted.
Waterproof papers are advantageous on some soil surveys.
They can be processed somewhat faster than the conventional
photographic paper and, if properly processed, their dimensional
stability is somewhat better. For soil surveys in warm humid
regions the waterproof paper has the advantage of absorbing less
Pens.-Pens should have points ranging from medium-fine to
fine. Pens with stiff firm points are much preferred to those hav-
ing soft flexible nibs. Unless used by an expert, pens with highly
flexible nibs spread, and lines are either too heavy or too light.
Such points soon lose their spring if abused and need to be dis-
carded. The stiffer, coarser pen lasts longer and permits more
uniform and consistent line work.
Erasers.-For cleaning soft pencil lines from aerial photo-
graphs art gum is usually satisfactory. Hard pencil lines can be
removed with a soft pliable eraser. Ink lines not removable with a
soft pliable eraser can be taken off by first dampening with alco-
hol or water and then erasing. Care must be taken not to let the
photographic emulsion become wet enough to break or tear with
erasing. Coarse or abrasive erasers should not be used on photo-
graphs. The emulsion becomes so scratched and broken that reink-
ing is difficult or impossible.
The sketchmaster.-The sketchmaster is a small instrument
used to reflect the image of the aerial photograph to a manuscript
map. The photograph is mounted parallel to the manuscript map
on a tripod-supported frame. The operator looks down through a
half-silvered mirror at the front of the instrument and sees the
image of the photograph superimposed on the manuscript map
(fig. 1). He can adjust the length of the three legs to correct for
tilt and difference in scale. The sketchmaster can be used for
sketching at scales ranging from one-half to twice that of the
Simplicity, compactness, and portability make the sketchmaster
an excellent instrument for use in field offices to transfer plani-
metric and soil data from the field sheets to a manuscript map. It
SOIL SURVEY MANUAL 63
Eyepiece --- \
Lens --Vertical photograph
Image of field sheet Manuscript
FIGTRE I.--Dingram showing the principles employed in a common type of
sketchmaster used for transferring map data from a photograph to a
may be used for overlays and other field sheets as well as for
Sketchmasters may be vertical or oblique. The vertical sketch-
master is used with vertical aerial photographs and the oblique is
used with oblique aerial photographs. Since the vertical aerial
photograph is used mainly in soil mapping, only the vertical
sketchmaster concerns us here. The same general techniques are
used with overlays of vertical aerial photographs or plane-table
In working with the sketchmaster, a framework of control is
first indicated on the manuscript map, and into this framework
planimetric detail is transferred from the photograph. This frame-
work assists the operator to orient his instrument and thus to
transfer the map data to their correct position on the manuscript.
The framework may consist of photogrammetric stations, or cul-
ture and drainage, along with established section lines or other
If the manuscript map is a standard topographic or planimetric
map, only soil boundaries and symbols, and revisions in culture
and drainage need to be transferred.
An operator uses a sketchmaster about as follows:
(1) After inspecting the aerial photograph to make certain that the
detail is clearly delineated, it is inserted in the frame so that it is
perfectly flat and all detail is shown.
U. S. DEPT. AGRICULTURE HANDBOOK 18
(2) The instrument is placed on the manuscript map. Looking through
the eyepiece, the operator orients the instrument so that the image
reflected from the instrument is near the correct position on the
(3) Among the lenses furnished with the instrument one is selected
that removes practically all the parallax at the scale to be used.
With a low rig, for example, a large numbered lens is used. If a
point on the manuscript moves in a direction opposite to that of
the eye of the operator, a smaller numbered lens should be selected.
(4) The instrument is adjusted for scale by lowering or raising the
frame on all three legs. The final leg adjustment is made with the
screw feet. Correction for any tilt in the photograph may be made
by adjusting the length of one or two legs.
(5) The detail on the manuscript should coincide with the detail reflected
to it from the aerial photograph.
(6) The detail on the aerial photograph is now ready to be trans-
ferred to its correct position on the map manuscript. The eye is
shifted slightly to bring individual controls into exact register as
the detail in their vicinity is being traced. When tracing a stream,
for example, the operator holds to the control on or near it. If
there is no control on a feature, a skilled operator can properly
orient nearby points in order to locate boundaries correctly. Trans-
ferring can be extended out to the edges of the vertical photographs.
Good light is required. The mirrors should not be touched with
the hands since the salt in perspiration decomposes the chemical
coating of the mirror and spoils it.
The soil scientist's most important tool is the humble spade,
supplemented by the pick and the soil auger. For exposing soil
profiles for morphological examinations, as in the initial work of
preparing a mapping legend, for sampling, or for photographing,
the spade is used almost entirely. For the more frequent routine
examinations of soils in mapping, the spade is generally but not
always superior to the auger. For example, where the chief differ-
entiating characteristic between two soil types or phases is the
depth to a deep underlying stratum of clay or is the color of the
substratum, a soil auger may be better than the spade, both faster
and more convenient. Perhaps the worst feature of the auger is its
destruction of soil structure, so important in classification and
identification. In dry, stony soils, the auger is difficult to use; nor
can the spade alone be used rapidly; and the pick becomes the
most useful tool. Whenever practical, the spade should be given
preference over the soil auger, especially in excavating the upper
part of the soil-the solum. Where the soil auger is used frequently
in identifying soils, some exposed profiles of the soil types should
also be examined in order to check the results.
Spades and picks.-For use in collecting samples, especially
after the preliminary excavation has been made, the flat square-
pointed spade (fig. 2, A) is most convenient. The best generally
useful spade, however, for ordinary use in mapping, is a modified
post hole spade (fig. 2, B and C). The sharp corners of the post
hole spade are removed for best results. The common tiling spade
tapers somewhat too much at the end, although it is a useful tool
for some soils and generally superior to the post hole spade for
SOIL SURVEY MANUAL
A B C D
FIGURE 2.-Soil-sampling tools: A, Square-pointed spade, especially useful
in collecting samples; B, side view, and C, front view of post hole spade,
the most generally useful sampling tool; and D, soil auger with extension.
gravelly soils. Where deep holes are required, as in examining irri-
gated Alluvial soils, the long-handled irrigator's shovel is useful.
It may be necessary to supplement this shovel with a heavy crow-
bar to penetrate dry cemented and compact layers.
The pick should always be at hand, especially for making holes
in hard, dry, stony, or gravelly soils. In some soils a small trench
pick will serve satisfactorily, but commonly a heavier pick with a
long handle is better. One prong should be sharply pointed and the
other made as a chisel. A heavy chisel-pointed bar is useful for
penetrating strongly cemented or indurated hardpans.
A geologist's hammer, or small hand pick, one end of which can
be used as a hammer, is also useful in examining rocks and the
soil in cuts along roadsides. For moist soils and those containing
many woody roots, a chisel-pointed hammer is better; whereas for
dry soils a sharp-pointed hammer is better.
Augers.-The screw, or worm, type of soil auger (fig. 2, C) con-
sists essentially of a 11 1.- or 11 .'-inch wood auger, from which the
cutting side flanges and tip have been removed, welded to a steel
rod or iron pipe with a crosspiece at the top for a handle. The
worm part should be about 7 inches long, with the distances be-
tween flanges about the same as the diameter, 11/, to 11/ inches. If
the distance between flanges is narrower, it is difficult to remove
the soil with the thumb. For ordinary use augers are 40 to 60
66 U. S. DEPT. AGRICULTURE HANDBOOK 18
inches long, with provisions for adding extra lengths for deep
boring. An auger for continual use is made solidly throughout,
and another extension auger is used for deep borings. In clay soils
an auger with a 1-inch bit may be more convenient than the larger
one. It is convenient to have a scale marked on the shaft of the
auger from the tip.
FIUzE :3*-Core type or soil auger: Left, a (lose view of the bit; right, a
view of the whole age', With extsions, arked t -inch intervals.
I 'i F
FIGURE :.--Core type of soil auger: Left. a close view of the bit; right, a
view of the whole auger, with extensions, marked at (;-inch intervals.
SOIL SURVEY MANUAL 67
Generally, the core, or post hole, type of soil auger shown in
figure 3 is better than the older screw type. The core type is espe-
cially favored in dry regions, and the screw type in wet ones. The
core type gives a larger and less modified sample. It works well in
loose dry sand and in compact soils. The cylinder is about 2 to 4
inches in diameter, commonly 31// inches. The cutting blades are
so constructed that the soil is loosened and forced into the cylinder
of the auger as it is rotated and pushed into the soil. Each filling
of the cylinder corresponds to a penetration of 3 to 5 inches.
Although both ends of the cylinder are open, the soil becomes
packed enough to stay in it while the auger is removed. If the
cylinder is only partly filled, or if the soil is very dry and sandy,
it may need to be tamped with a stick thrust through the upper
end of the cylinder before it will stay in the auger when pulled out
of the hole. Small cylinders are best for very sandy soils. A few
taps of the cylinder on the ground or on a board usually loosens
the soil for removal.
The core-type auger disturbs the soil, but less so than the screw-
type auger. A better view of soil structure, porosity, consistence,
and color can be had with the core auger, but even so, excavations
are necessary for proper morphological studies. The core-type
auger is not well suited to use in wet clay soils. Generally, with
soils that are naturally moist for much of the year, the screw-type
auger is faster.
Although soil augers are simple in design and somewhat crude
in appearance, considerable skill is required to use them effectively
in making dependable observations of the soil profile.
Peat sampler.-Examinations of deep deposits of peat are made
with a special sampler. Although several devices are used, the one
most common in the United States is the Davis peat sampler or
some modification of it, as shown in figure 4. The instrument
consists of 10 or more sections of steel rods, each 2 or 4 feet long,
and a cylinder of brass or duraluminum, approximately 14 inches
long with an inside diameter of three-fourths inch. The cylinder
is provided with a plunger, cone-shaped at the lower end, and with
a spring catch near the upper end. The sampler is pressed into the
peat until the desired depth is reached for taking a sample; then
the spring catch allows withdrawal of the plunger from its en-
closing cylinder. With the plunger withdrawn and locked in that
position, the cylinder may be filled with a solid core of the organic
material by a further downward movement. The cylinder protects
the sample completely from any contamination and does not de-
stroy its structure when the instrument is removed.
Beginning at the surface, samples of peat are taken consecu-
tively at intervals of 6 inches or 1 foot. The lengths of steel rods
used allow an easy estimation of the depth of each sample. For
very deep deposits, extra 2- or 4-foot rods are used. Each rod is
threaded at one end to screw into a small coupling on the reverse
end of another rod. For light work, the rods may be screwed and
unscrewed with pliers; for heavy work in deep deposits, small pipe
wrenches are used.
U. S. DEPT. AGRICULTURE HANDBOOK 18
FIGURE 4.-Peat sampler: A, The head closed, ready for pushing' into the
peat; B, the head extended, as just prior to taking a sample; C, one 2-foot
extension rod; and D, the top extension rod.
SOIL SURVEY MANUAL
Other sampling' tools.-Power augers, mounted on the rear of
a truck or on a trailer, some custom made and others obtainable
from manufacturers, are used in some soil surveys, either for spe-
cial studies or for cutting through cemented or very compact dry
soils. Some of these are of the core type, either similar to the hand
core auger already described or so constructed as to obtain an un-
disturbed core of a complete soil profile.- Others are of the screw
type. Further experience is needed with power augers. A custom-
built one in use is shown in figure 5.
FIGURE 5.-Custom-built power soil auger: A, Mounted on a pick-up truck in
position to operate; B, in position for transport; and C, close view of bit.
Another tool used little in routine soil mapping but of use in
collecting soil samples is the King soil tube, or a modification of it,
which consists of a long, narrow tube that can be driven into the
soil. It is used primarily in collecting soil samples for moisture
and bulk-density (or volume-weight) determinations. A short,
wide tube is used for collecting samples from soil horizons for
bulk-den-;ity determination. An angled cold chisel is convenient
for cutting out blocks of compact or cemented soil. An ordinary
SFor a description of a power aug'er see KELLEY, O. J., HARDMAN, J. A.,
JENNINGS, D. S. A SOIL-SAMPLING MACHINE FOR OBTAINING TWO-, THREE-,
AND FOUR-INCII DIAMETER CORE:S OF UNDISTURBED SOIL TO A DEPTH OF SIX FEET.
Soil Sci. Soc. Amer. Proc. 12: 85-87., illus. 1947.
70 U. S. DEPT. AGRICULTURE HANDBOOK 18
trowel is used for sampling thin horizons and for filling sample
sacks. A special trowel for this purpose consists essentially of an
ordinary curved garden trowel with about one-half of the blade
cut away (longitudinally) and sharpened. A straight-bladed steel
fern trowel is also a good tool. A handy tool for examining soil
profiles is a small steel pick of the type used by French workmen
in laying slate roofs. The head of this tool has a broad-bladed
chisel on one prong and a small hammer on the other. Finally,
every soil morphologist needs a strong knife.
FIELD TESTING APPARATUS
Several suitable field kits for pH determinations are available.
Where soils are very low or very high in pH, are highly organic,
or are salty, an electrical field kit is better than the simpler colori-
metric ones. Carbonates may be tested for with 10-percent hydro-
chloric acid solution in a small dropping bottle.
The sections on Soil Reaction and Estimation and Mapping of
Salts and Alkali in the Soil should be read and appropriate appa-
ratus obtained as required.
Besides these tests, manganese dioxide may be tested for by
using a 10-percent solution of hydrogen peroxide in a dropping
bottle. This is not a test for total manganese, and effervescence is
not necessarily correlated with toxic concentrations. The peroxide
test is useful in the field as a partial indicator of boundaries among
some lateritic and latosolic soils.
No kits for chemical "quick tests" for available or soluble plant
nutrients in soils are recommended. In some areas, particular
ones may be useful if well standardized by field plot tests.
PLOTTING AND ASSEMBLY OF FIELD DATA
The plotting and assembly of field data are discussed early in
the ManIial because of their importance to preparation for field
work, but some points may not be clear until later chapters dealing
more specifically with soil classification and mapping units are
AERIAL PHOTOGRAPHS IN SOIL SURVEYS
Characteristics of aerial photographs
The kinds of aerial photographs have already been described. First of
all, an aerial photograph is not a nmap but a perspective view of a portion
of the earth's surface. Like all perspectives, it does not present a true scale,
and precise measurements of distances and directions cannot be made on it.
In addition to the distortions of a perspective, there are those created by
tilt, differences in elevation, and inherent errors of photography. Yet in
contrast to maps made by ground methods, aerial photographs show more
ground detail, permit a three-dimension view of the features, and afford an
economical method for obtaining base material rapidly for large or inacces-
sible areas. From them, accurate planimetric and topographic maps can be
made. For most soil mapping, there is no better medium than the aerial
Oblique photographs are taken with cameras (often hand held) pointed
down at an angle such that the longitudinal axis of the camera forms an
angle of less than 90 with the ground. They are classified as (1) high
oblique, which show the horizon, and (2) low' obliqru, which do not show
the horizon. The high oblique shows a large area of the terrain in panorama,
whereas the low oblique shows only a small area of the ground. Although
obliques serve many purposes and are useful in reconnaissance surveys as
an aid to the identification of boundaries, they are not readily converted
to maps and are not so satisfactory for soil mapping as vertical pictures.
Vertical photographs are taken with fixed-level cameras pointed straight
down from the aircraft so that the longitudinal axis of the camera is per-
pendicular to the horizontal plane of the ground. Three broadly defined types
are (1) the continuous-strip photograph, (2) the multiple-lens, and (3) the
The strip photograph is a continuous-strip exposure. Strip photographs
may be taken from low altitudes at high speeds by synchronizing film
motion with the ground speed of the aircraft. In this way good pictures can
be taken with poor light. Strip photographs so taken have little use in soil
mapping because of their large scale and small coverage.
Multiple-lens photographs combine vertical and oblique camera angles.
The cameras usually have three, five, or nine lenses. One lens takes a vertical
view, and the others obliques. With a transforming printer, the obliques
are transformed to the plane of the vertical picture to produce a composite
vertical photograph composed of the center vertical picture and the trans-
formed obliques. The multiple-lens camera is widely used where rapid and
economical coverage of large areas at small scale is needed. Although the
pictures are occasionally used in soil mapping, they are not recommended
if single-lens pictures are available. The usually small scale, necessity for
transforming prints, and difficulties of map construction make them less
satisfactory than single-lens pictures. Multiple-lens photographs are also
obtained through the use of multiple cameras, arranged and mounted to
make vertical and oblique exposures. The tri-metrogon photograph is an
72 U. S. DEPT. AGRICULTURE HANDBOOK 18
Single-lens photographs, which are taken in a series of independent over-
lapping exposures, are recommended for soil mapping. They have convenient
size for field use and map construction, permit stereoscopic study, give
excellent detail of ground features and also have satisfactory ranges of
In discussing the use of aerial photographs in this Alunval, single-lens
vertical aerial photographs, made to the specifications of the Department of
Agriculture, are assumed unless otherwise stated. Where it is necessary to
use other types of photographs or single-lens photographs of lower standards,
the Cartographic Section of the Division of Soil Survey will advise the soil
scientists about methods to use and their specific weaknesses. Excessive tilt,
insufficient overlap, and other deficiencies may make it impossible to use
the pictures stereoscopically or to construct accurate maps from them.
Flight lines and overlap.-Most aerial photography in this country is flown
north and south. Flight lines are as near straight and parallel as possible;
they should not deviate from the true direction by more than 5 degrees.
Flight lines are usually continuous across the area, with the first and last
photograph on each flight line falling entirely outside the area boundary.
In line of flight, consecutive photographs should overlap an average of
60 percent, with no overlap less than 55 percent nor more than 65 percent.
Overlap in line of flight is referred to as cndlap. The overlap between ad-
jacent flight lines, or sidclap, should average 30 percent, with none less than
15 percent nor more than 45 percent. (See figure 6.)
/i /I\ /1\ /1\
SI\ "I / i\ /i\
/ \ / \ / \
/ / I \ i
/ // I /
60 percent endlap 30 percent sidelap
FIGURE 6.-Diagrams showing overlap in single-lens photographs.
Adequate overlap is essential for stereoscopic study in the field and for
the photogrammetric processes used in map construction. Where alternate
photographs-every other photograph in line of flight-are used, the overlap
of standard pictures averages only 20 percent.
Full stereoscopic coverage should le obtained for soil mapping, even though
the soil boundaries are plotted only on alternate photographs.
Scale.-The scale of aerial photographs is not always accurate nor uniform
like that of a good map. The scale varies between photographs because of
varying altitudes of the aircraft, differences in ground elevations, or tilt
of the camera. The sketch in figure 7 shows that a photograph taken at
camera station A will not le the same scale as a photograph taken at
station B because the aircraft is at different altitudes at the times of expo-
sures. Thus, the 20-acre field C will not measure the same as the 20-acre
field D because of the elevation differences within the photograph.
The scale given for photographs is the approximate average scale com-
puted from the mean altitude of the entire area flown, from that of a
SOIL SURVEY MANUAL
FIcrmun 7.-Sketch showing differences in scale at various heights. A photo-
graph taken at station A will have a different scale from one taken at
station D. Patterns of the two 20-acre fields at D and C have different
dimensions on the photographs.
/ I N
FIGURE 8.--Sketches showing distortion of a square in an aerial photograph
because of tilt: A, normal; B, with tilt.
93503.1 -51 -6
74 U. S. DEPT. AGRICULTURE HANDBOOK 18
FIGURE 9.-Air photographs for the same area: Upper, normal; lower,
SOIL SURVEY MANUAL
fraction of the area, or from a specified datum plane. Photographs flown
for the United States Department of Agriculture do not deviate more than
5 percent from the average scale. Most of this aerial photography has a
scale of 1:20,000 (:.168 inches to 1 mile) from which satisfactory enlarge-
ments or reductions can be made.
Tilt.-When the plane of the camera is not level the resulting photograph
is tilted. The greater the tilt, the more the objects in the photograph are
distorted in shape and size. Figure 8 illustrates such distortions in the
shape and scale of a square field. Figure 9 shows aerial photos for the same
area, normal and with tilt. Excessive tilt may sometimes be detected by
comparing images in overlapping photographs. In standard photography
used by the United States Department of Agriculture tilt does not exceed 5,
nor average more than 2 in a 10-mile flight line, nor average more than
1 for an entire area.
Crab and drift.-To maintain a true flight line in the presence of side or
quartering winds, it is frequently necessary that the photographic aircraft
fly at an angle to the flight line. The camera is rotated to compensate for
the angle of flight. Failure to do so results in crabbcd photographs as illus-
trated in figure 10,A. Drift, a special form of crab, results when exposures
oriented to the flight line continue to be made even though the aircraft has
drifted from the flight line. Edges of successive photos are parallel but side-
stepped, as sketched in figure 10,B?. Standard specifications do not permit
crab to exceed 10 from the true flight line in any two or more pictures.
<~ T ~--
FIGURE 10.-Sketches showing irregularities in aerial photographs: A, from
crabbing; B, from drift.
Displacement.-Tf a vertical photograph is in a plane parallel with a section
of flat ground, the relationships between images in the photograph and
objects on the ground are similar. If the ground objects are not all in the
U. S. DEPT. AGRICULTURE HANDBOOK 18
same horizontal plane, however, the photographs do not present objects in
correct relationship to one another. This difference is called disploccelmirt,
illustrated in figure 11. It can he seen that displacement is inward, toward
the center of the photograph, for objects below the datum plane; and out-
ward, from the center, for those above the datum plane. An object appearing
at the center of an untilted photograph is not displaced, regardless of relief.
Displacement and tilt are so interrelated that the layman finds it is impos-
sible to differentiate between them. The vertical datum plane is considered
only when heights of objects or differences in elevation are to be measured,
as in the construction of a topographic map.
FIcURE 11.-Sketch illustrating displacement in aerial photographs. Note
building A in the valley and watertower D on the hill. A vertical line
through the building intersects the datum plane at A'. Since the building
is below the datum plane, it will appear at point a on the aerial negative
rather than at its true ground position o'. Similarly, the watertower, above
the datum plane, will appear at b rather than at its true position b'.
Inherent errors.-Tnherent errors arise from the physical limitations of
materials and instruments used in taking and preparing aerial photographs
and are more or less common to all photographic processing. They result
from improper grinding or other imperfections in lenses, curvature of the
lens field, inefficient shutters, and expansion and contraction of photographic
S \ VERTICAL
SOIL SURVEY MANUAL
film and papers. These errors are reduced to the minimum in modern aerial
photography where only precision mapping cameras, excellent laboratory
equipment, and special aero film and paper are used.
Code numbers.-Aerial photographs are marked when taken and processed
to permit indexing and rapid selection. Although organizations indicate
different information in various ways on their aerial photographs, the fol-
lowing are usually shown: (1) Date of flight, (2) time of day, (3) owner
of film, (4) scale of negative, (5) project or area, (6) film roll number,
and (7) exposure number. Some also show altitude, focal length of camera,
type of camera, and the like.
Each aerial photograph made for the United States Department of
Agriculture bears a code letter designating the project or area and individual
numbers to designate the roll of film and exposure. These are in the north-
east corner for north-south flights and in the northwest corner for east-west
flights. The code number for the area is limited to three letters; the roll
of film is indicated by number, beginning with one and continuing unbroken;
and numbers indicate the exposures, beginning with one for each roll of
film and continuing unbroken. For example, in the designation ABC-46-122,
ABC indicates the county or area, 46 the roll of film in that county or area,
and 122 the exposure in that roll. In the adjacent corner are numbers for
the month, day, and year the exposure was made. On the first and last
exposure in each roll of film appears the abbreviation for the organization
owning the film, the approximate scale of the negatives, and the time of
day the exposures were made. The organization abbreviation and approximate
scale precede the usual area symbol, as BPI-1:20,000-ABC-46-122. In the
adjacent corner, immediately following the date, the time of day is placed,
Photo indexes.-Aerial photographic indexes are prepared for large areas.
Without an index the user of photographs is seriously handicapped in
selecting the photograph for a specific area or in locating adjacent photo-
graphs in adjoining flights. Photo indexes are prepared by laying the over-
lapping photographs so that the index numbers of each print are visible.
Standard specifications usually require the index to be in sheets 20 by 24
inches at an approximate scale of 1:63,360. The soil survey party should have
the photo index of the area to expedite the location of individual photographs.
Although individual aerial photographs are flat in appearance, overlapping
pairs can be viewed under a stereoscope and the topography of the ground
becomes apparent: hills and valleys appear, buildings and trees stand up,
and the slight depressions of drainage can be seen. Thus viewed, the aerial
photograph looks like a detailed relief model. The soil scientist can study
the ground before going into the field. Drainage and trails that are obscure
on the flat photographs can be outlined in advance. Travel routes can be
selected. Stereoscopic study of the pictures, both before and after the
mapping, helps him to see the relations between kinds of soil and land forms.
Theory of stereovision.-In normal vision, the observer sees objects in three
dimensions, namely length, width, and depth. The ability to see depth depends
on sight with two eyes, each at an equal distance from the object but
viewing it from a different position, or angle. Each eye registers a slightly
different image. These images are fused or combined by the optic nerves and
brain to give depth perception or a third dimensional view of the object.
The distance between the eyes is so short that the angle and difference
becomes so small at great distances that it is difficult to register depth
When viewing two overlapping aerial photographs under the stereoscope,
one sees the same ground area from widely separated positions. The right
eye is viewing the area in one photograph, the left eye the same area in
another photograph. The effect is the same as if a person were viewing the
area with one eye located at one camera position and the other eye at the
next camera position. The brain so fuses the images that one sees the relief
in the photograph, or the third dimension.
U. S. DEPT. AGRICULTURE HANDBOOK 18
The average person with normal vision should have little difficulty with
stereoscopic study of aerial photographs. Occasionally -a person with appar-
ently normal vision is unable to use the stereoscope. This may be expected
of older persons whose eye muscles are not flexible. Some feel eyestrain
when first using the stereoscope.
Stereoscopic vision requires some practice. At first it may be difficult to
adjust the photographs and fuse the images; yet after practice this can
be done rapidly with little or no eyestrain.
Types of stereoscopes.--Stereoscopes are constructed on two basic prin-
ciples. Those most commonly used in the study of aerial photographs are
(1) the mirror type, which utilizes the principle of reflection, and (2) the
lens type, which makes use of the principle of refraction. A third type, less
commonly used, is the prism stereoscope. In this type prisms serve as
reflectors, much as mirrors do in the mirror stereoscope. Designs of all
types vary widely.
The mirror stereoscope has four mirrors fastened in a frame and arranged
to transmit the photographic image to the eye by reflection (fig. 12). Since
these stereoscopes are usually large and bulky, they are not easily portable
and are used mainly for office work where plenty of table space is available.
Some mirror stereoscopes are designed to fold up and fit in a small case
that can be carried in a large pocket. Even these are too bulky to carry
in the field while mapping.
Mirror Mj 0 Mirror
FIGURE 12.-Sketch showing the essentials of the design of a mirror
The mirror stereoscope gives an image nearly free of distortion. It has
a wide field of vision-wide enough for one to view an entire photograph.
The wide separation of the mirrors allows the photographs to be viewed
without overlapping, which makes adjustment and fusion of the picture
simple. Many instruments have a horizontal adjustment of the outer mirrors
which allows them to be placed at various distances from the eyepiece. With
this adjustment, larger scale photographs can be viewed than with the
conventional lens-type stereoscope. Owing to the great optical distance
between the eye and the photograph, the fused image appears to be reduced.
SOIL SURVEY MANUAL
This is a disadvantage in studying fine detail, especially on small scale
photographs. The disadvantage may be overcome by fitting the stereoscope
with magnifying lens, but this increases the size and cost of the instrument.
The mirror stereoscope is especially good for the office study of aerial
photographs. It is simple for the beginner to use and requires little practice.
Lens stereoscopes have two magnifying lenses mounted in a frame and
supported on a stand so that the photographs are viewed directly through
the lenses, or eyepieces. The lenses are ground so that the lines of sight are
bent outward (fig. 13). These instruments are usually small, compact, and
light. Many are designed for field use and fold into a small unit that can
be carried easily by the soil mapper in the field. Most use, however, is in
the field headquarters.
FIGURE 1'.-Sketch showing the essentials of the design of a lens stereoscope.
The thin edges of the lenses are inside.
The lens stereoscope gives a distorted image. It has a small field of
vision, and only part of the photograph can be viewed at one time. The close
spacing of the lenses, combined with direct vision, makes it necessary to
place the photographs very close together or even to overlap them. Thus
adjustment of the photographs and fusion of the images are somewhat
difficult. For the same reason, large-scale photographs cannot be viewed
satisfactorily except at the margins. Despite these disadvantages, the lens
stereoscope is a useful tool for the soil scientist.
The lens stereoscope magnifies, which is a definite advantage, especially
when studying minute detail or very small-scale aerial photographs. It
emphasizes the relief, which is helpful in viewing nearly flat terrain.
The lens stereoscope is helpful for the field study of aerial photographs
where the scale is small enough to permit its ready use. Appropriate models
are light, compact, and relatively cheap.
Care of stereoscopes.-Stereoscopes are generally of rugged construction
and will withstand a reasonable amount of hard use; but they are optical
instruments and should be treated accordingly. "First-surface" mirrors are
used in stereoscopes. In these the silver is applied to the front of the glass
and not to the back. The silver coating is highly susceptible to scratching
80 U. S. DEPT. AGRICULTURE HANDBOOK 18
and corrosion and should not be touched. All first-surface mirrors should be
protected with a soft cloth or chamois covering when not in use.
A first-surface mirror may be cleaned with soft clean cotton and alcohol.
The cotton needs to be free from any grit that might scratch the silvered
surface. The silvered surface is wiped gently with just enough pressure to
remove the dirt. Fingerprints should be cleaned off immediately, since their
residues corrode the mirror.
The lens type of stereoscope should be cleaned and cared for like a pair
of glasses. Stereoscopic lenses are usually ground with one side thinner than
the other in order to reflect the light rays outward. Such lens must be placed
in the frame with the thick edge outward and the thin edge inward.
Use of the stereoscope.-To use the stereoscope in studying aerial photo-
graphs one must first acquire the knack of adjusting the photographs and
accustoming the eyes to stereoscopic vision. This ability may be acquired in
One of the simplest methods is to place a small cross on two separate
sheets of paper. With each sheet of paper under the lens, or mirror, of the
stereoscope, one may look directly through the eyepieces of the stereoscope
and focus the eyes on the crosses. Unless the crosses by chance are fused,
one sees two crosses. After the eyes are focused, one sheet is held firmly
and the other moved slowly. The crosses move either nearer or farther
from each other. The sheets are slowly shifted until the two crosses coincide
and appear as one. During the operation, the eyes are not shifted nor the
Once the crosses coincide, the sheet is moved until the image separates
into two crosses again; then again the sheets are shifted until the crosses
appear as one image. This practice is continued until "fusing" can be done
rapidly. When the crosses are fused, the approximate location of the sheets
with reference to the lens or mirrors is noted. The sheets are removed and
then replaced to try focussing the eyes and fusing the crosses rapidly.
When the operation can be performed quickly and accurately, one is ready
to attempt stereoscopic vision with two aerial photographs.
To start, one may select two stereo-pairs of terrain with moderate relief
and a distinct pattern of ground features. The photographs should be of
equal tone and scale. The center of the photograph-its optical center-is
located at the intersection of lines drawn between collimation marks, usually
appearing as small ticks at the center of each margin, and marked with a
cross. The picture centers are transferred to the overlap area in each ad-
joining picture, and the two crosses on each picture connected with a line.
The photographs are placed under the stereoscope with the overlapping
detail approximately in coincidence and the lines on the photographs parallel
to the eye base. The shadows should fall toward the observer and both
photographs need to be well and uniformly illuminated.
The photographs are shifted horizontally and adjusted until the crosses
and connecting lines are fused. Then the relief can be seen. One of the
photographs should be shifted until fusion is lost, and later recovered. With
practice, images may be fused rapidly. As skill develops, the observer fuses
the images by observing the physical features and less by watching the
crosses and connecting lines.
After skill in these exercises has been obtained, it is time to try two
photographs without the centers marked and connected. Two such pictures
are placed under the stereoscope with the index finger of each hand just
under the same selected physical feature in the overlap area on each photo-
graph. The eyes are focused and the pictures shifted until the fingers
appI~ ximately coincide. The fingers are moved away and the images slightly
separated. Then they are adjusted until they again coincide. This practice
needs to be continued with other stereo-pairs. Once the knack of placing the
photographs and adjusting them until they fuse has been acquired, the
operator is ready to use the stereoscope in the study of aerial photographs.
Lenses need to be focused properly. Many stereoscopes have an adjustment
for varying the spacing of the eyepieces, so that they can be separated to
the correct interpupillary distance of the observer's eyes.
SOIL SURVEY MANUAL 81
Generally, on aerial photographs man-made features appear in geometric
patterns-wi.h straight lines, sharp angles, and circles.
Natural features, generally, have irregular and curved lines, as in twisting
streams, curving shore lines, and the like.
For interpretation of size one needs to know the approximate scale of the
photograph. A round image may represent a silo on a large-scale print, and
one of the same s:ze, a large gas storage tank on a small-scale print. Size
is sensed by comparison among the ground objects.
The tone, or shade, in which various features appear on an aerial photo-
graph is due mainly to the amount of reflected light. The amount of reflected
light depends upon the texture of the surface of the object and the angle
at which the light is reflected. An object that reflects a large amount of
light appears in a light tone on the photograph. If little light is reflected,
the object appears dark.
Because of differences in the angle of reflected light, the tone of an
object may he different on two consecutive photographs, especially if the
surface is smooth and a good reflector of light. Thus, in one photograph,
with the light rays reflected from the water to the camera, a water area
appears light; in an adjoining photograph, with the angle of reflection away
from the camera, the same body of water appears dark. Most natural fea-
tures, however, reflect light in all directions and appear in intermediate
tones, for some of the reflected light finds its way to the camera lens.
For stereoscopic study, the light should hIe good and each photograph
should he equally illuminated, but without glare. Where possible, the observer
should face the source of the light. Of course, these lighting conditions
cannot always be arranged in the field.
The photographs are placed under the stereoscope in such a way that the
one taken to the left of the overlap area is viewed by the left eye, and the
one to the light of the overlap by the right eye. If the position of the
photographs is reversed, and the left eye views the right photo and the right
the left, the image of relief appears in reverse. In such an arrangement,
points of low elevation appear high, and points of high elevation low. This
is commonly called a pseuduscopic image.
Photographic characteristics.-Certain characteristics of aerial photographs
are the basis of stereoscopic interpretation. The most important ones are the
shape and size of features, the tone in which the features appear on the
photographs, and the shadows cast by the features.
The shape of features is important in the interpretation of ground detail
from aerial photographs. The observer needs to study the shape of ground
objects as they appear on the vertical photographs in comparison with how
the sameo features look on the ground. Frequently the tone of objects appears
darker or li ghter in photographs than their contrasting ground colors
Shadows on aerial photographs often reveal the size, shape, and identity
of objects. The shadows suggest the heights of objects, which are not revealed
by the horizontal dimensions alone. A one-story building, for example, may
look like a five-story one in the picture except for the shadow. But shadows
can be deceptive. If the ground under the object slopes abruptly, the shadow
may be distorted. The height of the sun at the time of exposure also affects
the length of the shadow. Yet many objects with little width, like fences,
flagpoles, and chimneys, are difficult to identify except by their shadows.
Interpretalion.-Most field scientists become proficient in photo inter-
prctation by using the photographs in the field where opportunities are con-
tinually offered to compare ground features with their photographic images.
Study of the photographs of the area to be covered a day or so in advance
can be helpful. The accuracy of interpretations is checked by observations
of the ground detail. Images that are unidentifiable in the office can be
identified in the field. With such practice, the soil scientist can rapidly train
himself in aerial interpretation. It must always he recalled, however, that
accurate photo interpretation depends on familiarity with ground conditions.
Ability to interpret pictures accurately in one area is not necessarily fol-
lowed by similar accuracy in another area with different conditions.
U. S. DEPT. AGRICULTURE HANDBOOK 18
The time of day and season of the year in which photographs are taken
influence interpretation. In order to have good light, most photography is
taken under ideal weather conditions and during the middle part of the day.
The length of shadows naturally depends upon the height of the sun. Shadows
on photographs taken in summer are shorter than those on photographs
taken in winter. Similarly, shadows appear much longer on photographs
taken a few hours before or after noon than on those taken at noon.
Features appear differently on aerial photographs in the different seasons.
Cultivated fields vary from season to season. In wet seasons, streams appear
large and many small ponds may be visible. In the dry season, the same
area may have no ponds and the streams may be dry beds. During summer,
deciduous forests present a mass of treetops that obscure the ground detail.
In winter, pictures of the same area show a confusion of tree trunks,
emphasized by shadows, and trails, small drains, and other ground detail
are clear. Snow on the ground obscures much of the detail. The experienced
photo interpreter takes all of these factors into consideration when studying
A few of the characteristics of some major features as they appear in
aerial photographs may lie helpful in acquiring skill in photo interpretation.
With experience, the soil scientist can broaden his information.
Stfricam.-Streams are usually identified by their irregular widths and
winding courses, frequently emphasized by the growth of brush and trees
along the banks. In heavily wooded areas, small streams are difficult to
detect. Water in stream beds is suggested by dark or light lines, depending
on the angle of the reflected light. Dry stream beds are easily recognized
in the open and usually appear in light tones.
Docdiro of water.-Ponds and lakes appear lighter or darker than the
adjoining shore, depending on the reflected light. Furthermore, they are
flat. The shore lines are sharply defined and appear as irregular outlines.
One end of a large lake in a photograph may appear light and the other
Mar.shcs.--Swamps and marshes have a blurred appearance. Many display
very winding channels, or small bodies of open water. Very wet areas or
those partially covered with water usually appear darker than the surround-
Forests and bruish.-Wooded areas appear as dark masses with irregular
outlines. The intensity of tone for deciduous cover varies with the season.
In summer the tone is very dark; in winter it is lighter. Coniferous forests
appear dark in tone, regardless of the season. Brush areas have a dark tone
in summer and a lighter one in winter, but shadows are lacking. Stereoscopic
inspection suggests the height.
Cftirat(n r arcars.-Cultivated fields are readily identified by their con-
trasting tones and their boundaries. Many field edges are well defined by
fences, hedges, trails, or roads. Terraces, contour strips, and other patterns
Some crops can be identified by planting patterns and tone or shade.
Fields with heavy standing crops or grass appear dark. Fields from which
crops have been harvested recently appear lighter. During harvest of crops
such as hay, wheat, and corn, fields acquire a distinctive pattern. Shocks
appear as dark-colored, regularly spaced dots against a lighter background.
Because of the rough surface and damp soil, freshly plowed fields are
usually very dark. The pattern of plowing is frequently visible on the
photograph. Orchards, vineyards, and similar plantings are readily identified
by their distinctive spacing.
Roa(s, frails, and path.s.-Roads usually appear as light lines. Cement
concrete roads have well-defined edges and appear light except for streaks
of oil drop in the center of each lane. Bituminous concrete or other black-
surfaced roads may seem dark. Most improved roads are identified by long
straight stretches, gentle curves, and regular width. Unimproved roads are
more irregular, have sharper curves, and vary in width.
SOIL SURVEY MANUAL 83
Trails meander and often follow the contour. Paths are even more indis-
tinct and irIegular. If used a great deal, paths and trails appear as light
The appearance of roads, trails, and paths changes with the season
because of shadows cast by trees and partial covering by overhanging
Railioads.-Railroads appear much like roads on the photographs but are
usually darker and narrower. They have long straight stretches and smoother
curves than loads. The roadbed material affects the tone appearance of the
railroad. Large cuts and fills, water tanks, spur lines, and stations are
distinct along the right-of-way.
Dildi(ngs.--Buildings are readily identified on aerial photographs. Their
size is suggested by their relation to the scale of the photograph; and
comparative heights can be estimated from the shadows. Isolated buildings
often have roads or trails leading to them. Individual buildings in groups
may be indistinct because of the collective shadows.
Laid form.-With practice, land form may he suggested from the photo-
graphs. In fact, land forms may be clearer in aerial photographs than on
any map or from the ground. As a start, the soil scientist may consult
some of the standard texts on this subject.'
USING THE AERIAL PHOTOGRAPH
Techniques of using aerial photographs in soil mapping differ
from those of using maps because measurements of courses and
distances are less precise. Even though the vertical aerial photo-
graph looks like a map, it is not an accurate plan of the ground
Stereoscopic interpretation and delineation.-To begin with, the
soil survey party should study the photographs of the survey area.
Each mapper needs to be familiar with the film-roll and picture
numbers, the sequence of flight lines, and direction of flights.
Study of the photo index will also serve to give a view of the whole
area and of the conditions to be met during the survey. Such study
can be helped by laying out photographs for parts of the area on
a table top as a sort of rough unassembled mosaic.
Once the survey is under way, it is helpful to study the photo-
graphs with the stereoscope before going into the field. Preliminary
interpretations of features can be made and the scientist can
familiarize himself with the ground conditions.
It is frequently helpful to delineate in advance certain ground
features. Drainage can be accurately plotted on the photograph.
The plotting of drainage features in heavily wooded areas is espe-
cially helpful to field orientation. Trails and obscure paths can be
marked for reference. Possible places where streams may be
crossed and routes through rough terrain can be tentatively se-
lected. Buildings that might otherwise be overlooked can be
marked for checking in the field. Such features as gullies, areas
of eroded soil, pasture or idle land, and forests can be tentatively
outlined and the time of scientists in the field thereby conserved.
Delineations should be made in pencil, to be inked after confirma-
tion in the field. Rivers, lakes, and other prominent water fea-
'See, for example, SMITI, H. T. U. AEIIAL PHOTOG'IA'llS AND THEIR
APPLI(ATIONb. 372 pp., illus. New York. 19:3.
U. S. DEPT. AGRICULTURE HANDBOOK 18
tures can usually be inked in advance. Clearly defined stream
courses can be inked as dashed lines, and after field inspection the
dashes can be closed, or dots inserted, to indicate either perennial
or intermittent streams.
Match lines and matching.-A match line is an arbitrary line
drawn in the overlap area of a photograph to serve as a boun-
dary for the mapping on the photograph and is to be matched by
a similar boundary drawn through identical points on the adjoin-
When stereoscopic pairs of photographs are used in mapping
soils, the match lines should be placed to limit the plotting of data
to the central parts of the pictures where distortion is least. If
alternate photographs are used, match lines must be placed near
the outer limits of the photographs in the narrow margin of over-
lap. Necessarily, a match line so placed includes the least accurate
outer edge of the photograph. Thus using alternate pictures may
increase the difficulty of plotting soil boundaries accurately, of
matching soil boundaries from one sheet to another, and of trans-
ferring soil data to the base map, and decrease the accuracy of
Match lines can be placed on photographs either with or without
the stereoscope. In the stereoscopic method, a line is placed ap-
proximately midway in the overlap area of a photograph. This
photograph and its adjoining mate are placed under the stereo-
scope, the images fused, and the match line transferred to the ad-
joining picture. This process is continued through the line of
flight and between adjoining flights.
In placing a match line without the stereoscope, two distinguish-
able features are selected along the outer edge of the area to be
mapped on the photograph and connected by a straight line. The
same features are identified on the adjoining photograph and con-
nected. The process is continued throughout the area.
If it is helpful in soil mapping, the match lines may follow some
prominent ground feature, like a road, railroad, or river, even
though it is irregular or curving. In sectionized areas, match lines
may coincide with the land lines.
The colors of match lines should contrast with those of other
lines. Green ink is frequently used for match lines.
Although the placing of match lines requires time, they are a
necessity for good mapping. They avoid mapping of duplicate
areas on adjoining photographs, facilitate the transfer of soil
boundaries from one photo to another, and simplify the carto-
graphic transfer of soil data from the aerial photograph to the
Soil boundaries and other mapping should be broken sharply
and precisely at the match line when inked. Soil symbols should
be kept within the match line if possible.
The matching of the mapping on one photograph with that on
another can be done in several ways. The mapped photograph and
an adjoining unmapped photograph can be placed under the stereo-
scope and the images fused. The mapping along the match line
SOIL SURVEY MANUAL
of the completed photograph can then be transferred stereo-
scopically to the adjoining unmapped photograph, although this
is not good practice. It is better to join sheets aftcr mapping as
a check on the uniformity of the work being done by individual
members of a soil survey party.
If adjoining photographs are at the same scale, a strip of trans-
parent paper or plastic can be placed along the match line of a
mapped photograph. The marginal mapping is marked on the
transparent strip. This strip is then placed along the match line
of another mapped photo for checking or of an unmapped photo
for transfer of the soil boundaries to the match line.
Another method, which is particularly useful when adjoining
photographs vary in scale, is to transfer boundaries by reference
to photographic images. Along the match line one observes the
relationship of the soil boundaries to such features as isolated
trees, clumps of bushes, field corners, and the like. The same
features are located along the match line of the adjoining photo-
graph and the boundaries checked or transferred to their same
position of relationship to that feature. Difficulty is had if dis-
tinguishable ground objects are few.
One could scarcely overemphasize the need for care in match-
ing the mapping on one photograph with that on others, both for
joining of lines and for checking of the classification. Roads and
streams need to be continuous from one photo to another. Special
care is needed at the corners where four photographs come to-
gether. Without a systematic method, it is easy to make errors that
will make later interpretations of the mapping difficult or
A record should be maintained of the matching of adjoining
photographs. This is especially useful where a number of soil
scientists are working in the survey area. A transparent overlay
over the photo index makes a good means of keeping such a record.
As the photographs are matched, the overlay can be marked.
Some place the letters N, S, E, and W on each sheet and cross
them out as the sheet is joined on the north, south, east, and west.
Inking on the photograph.-After completing the survey on a
photograph in pencil, with boundaries matched to previously
completed and adjoining photographs, the field sheet should be
inked. For clarity and checking, it is helpful to ink the three major
classes of features in contrasting colors that have good photo-
graphic qualities. For example, roads, railroads, buildings, and
other cultural features may be in red, drainage features in blue-
black, soil boundaries and symbols in black, and section lines and
numbers in red or green to avoid confusion with roads.
Each group of' features can be inked in a separate operation.
Culture can be inked first in red, for example, and the classifi-
cation of roads and other features checked. I)rainage can then be
inked in a blue-black and inspected to see that individual drains
are properly joined, matched, and classified. Soil boundaries can
be later inked in black. When inking the soil boundaries, it is best
to close each individual area as one proceeds, or to ink the soil
U. S. DEPT. AGRICULTURE HANDBOOK 18
boundaries up to some specific line, like a road or field boundary.
As a soil area is closed, the symbol should be placed as near the
center as practicable. If the soil area is long or irregular, additional
symbols should be added for easy reading of the mapping, but no
more. Soil symbols should be placed to be read from the same
direction throughout the survey and should be approximately
parallel. Where soil areas are so small that the symbol must be
placed outside the area, it must carry a pointer to the area to
which it applies. Place names are usually the last to be inked.
These can be in black or in the same color as the feature to which
they apply. By leaving them to the last, they can be placed where
they will not obscure soil symbols and other detail. Place names
need to be arranged in ways that leave no question about which
features they designate. Names of railroads, rivers, and other fea-
tures that continue across a number of photos need not be re-
peated on each one-only enough for clarity. Care should be used
in the placing of stream names so that no confusion arises as to
which branch of a stream a name applies.
The inking of the major features on a soil map separately, and
in contrasting colors, permits the checking of each group of
features individually. This usually results in fewer mistakes. The
colors also facilitate the interpretation of the original field sheet
by other users. If all plotted data are inked in one color, the field
sheets appear congested and it is harder to check and to interpret
Inking on overlays.-Inking on overlays is done in the same way
as inking on photographs. Opaque inks should be used, as overlays
are frequently reproduced by direct printing methods. If standard
waterproof drawing inks are used on plastic overlays, they should
be coated lightly with a plastic spray to seal in the ink and pre-
Orientation of the aerial photograph.-One of the advantages
of the aerial photograph is the rapidity with which it may be
oriented in relation to the local detail and the unusual ease with
which the soil scientist can locate his position on the photograph.
Normally in field use, the photograph is oriented by locating fea-
tures on the ground having images readily identifiable on the
In relatively flat areas with scanty detail, orientation is more
difficult. In some places it is helpful to mount the photograph on
a plane table set up over an identifiable point. The photograph
may be oriented toward a second identifiable point with the
plane table oriented by its compass. Then short traverses may be
run in the area. Since variations of scale within the photograph
make some difficulty, it is necessary to tie the traverse to all iden-
tifiable landmarks along the route. Long unbroken lines of trav-
erse are rarely necessary. Nearby rather than distant features
should be used for orientation because of scale variations within
SOIL SURVEY MANUAL 87
Heavily wooded areas present problems in both orientation and
location. These can sometimes be overcome by running short
traverses into the wooded area from the identifiable features on
the outskirts of the area. Stereoscopic delineation of drainage fea-
tures on the aerial photograph before going into the field often
helps a great deal, and stereoscopic study of the relief while in
the field contributes to orientation and location in wooded areas,
especially in hilly districts.
Sometimes it is necessary to carry the orientation forward from
one photograph to another. This is done by overlapping the photo-
graphs. The photo centers are first marked by drawing intersect-
ing lines from the tick marks on the sides of the photographs and
transferring the center of the rear photograph to the forward
photograph and that of the forward photo to the rear one. The
photographs may then be laid over each other so that the centers
are superimposed in the overlap area. With a compass, a magnetic
north line may be placed on a photograph that has been properly
oriented by identifiable features. Then the north line is trans-
ferred to adjacent overlapping photographs either by association
of identifiable features or stereoscopically, and the second photo-
graph is oriented by using the compass.
The aerial photograph may be oriented with respect to true or
magnetic north when it is used with a detailed map of the area.
Two matching and identifiable points should be selected on both
the photograph and the map, preferably along a line near the cen-
ter of the photograph. The compass bearing of the line through
the points on the map can be measured with a protractor and the
photograph oriented accordingly. When the orientation is made
with a compass, the points selected should be easily identifiable
on the ground. Unless the land is too heavily wooded, a plane table
with compass is better than a hand compass.
Shadows on the aerial photographs serve to give rough compass
orientation provided one knows the time of the year and day at
which the photograph was taken.
Plotting soil boundaries.-Plotting of soil boundaries is largely
a matter of keeping on. -. I properly located with relation to the
detail of the photograph and drawing the soil boundaries in rela-
tion to the identifiable images on both the photograph and the
ground. Keeping himself accurately located on the picture is the
first requirement of the soil mapper. Scil boundaries are plotted
in relationship to easily identified landmarks, such as field boun-
daries, streams, buildings, edges of forests, isolated trees, roads,
and similar features. With abundant detail, compass orientation
and measurement of distances are not necessary and the soil
scientist is not tied to a traverse. He is free to move about and
examine the soil types as necessary. As measurements are not
necessary, the difference in scale within the photograph or be-
tween photographs are of little or no significance.
New features that need to be mapped and that have been estab-
lished since the photos were taken should be located by survey
rcsecting: intersecting lines, as nea'lv as possible at right angles
U. S. DEPT. AGRICULTURE HANDBOOK 18
to each other, from identifiable positions, locate a new position.
Measured distances from identifiable features will give the loca-
tion of such features. Usually the intersection of two measured
distances will be accurate enough for this purpose.-
When one has become thoroughly familiar with the soils of the
area and their relationships to the features of the landscape, many
soil boundaries may be visible on the photographs. These bound-
aries can be plotted on the photographs and verified as the
On photographs of heavily wooded areas or others with few
identifiable features, it may be necessary to measure directions
and distances in order to plot the soil boundaries accurately. This
is best accomplished by using the photograph on the plane table.
If the photograph is oriented by the compass and its scale is known,
measurements of directions and distances are largely a matter
of correcting for distortion. Every attempt should be made to tie
to identifiable images as often as possible and to use the local scale
for that portion of the photograph when measuring distances.
Corrections.-Corrections on aerial photographs need to be
made carefully. If the photographic emulsion is broken or
scratched, ink will run and smear so that symbols and lines will
not be legible. Corrections of ink lines should be made with a soft
eraser or with cotton and alcohol. If a photograph is so damaged
that it cannot be reinked legibly, it is best to superimpose a thin
sheet of transparent plastic over the area and reink the data on
this. The plastic overlay should be securely fastened to the
HORIZONTAL CONTROL FOR AERIAL PHOTOGRAPHS
To transfer the soil boundaries and symbols from an aerial
photograph to a compilation base, a planimetric map of good qual-
ity is required. In areas having no adequate map, it is necessary
to construct a base from the aerial photographs.
The scale of the map, its accuracy, and orientation and place-
ment on the earth's surface depend on horizontal ground control.
This control is the framework of a map and comparable to the
foundation and steel frame of a large building. After completion,
the frame is hidden from view.
A horizontal ground-control station is a precise point on the
earth's surface, the position of which has been accurately de-
termined by field survey methods in relation to certain parallels
of latitude and meridians of longitude. Many such stations are
scattered throughout the United States. These have been and
are being established by the United States Coast and Geodetic
Survey, the United States Geological Survey, the Corps of En-
gineers of the United States Army, the Lake Survey, the Missis-
sippi River Commission, and, to a limited extent, by private map-
ping and engineering firms. The positions and descriptions of
these stations are readily available from the records of the estab-
See also Appendix I. Map Preparation with the Plane Table.
SOIL SURVEY MANUAL
Distribution and extent.-Soil survey parties are not equipped
to establish ground control and depend upon and use the control
already established by other agencies. Using these control sta-
tions as a base, the Cartographic Section of the Division of Soil
Survey establishes photogrammetric control points between
widely spaced ground points by a process commonly known as
radial triangulation. These photogrammetric control points (also
called supplemental control) are then used on the map base for the
proper orientation and placing of each aerial photograph in order
that all culture, drainage, and soil boundaries may be shown in
their true positions on the soil map.
The density of established horizontal ground controls in the
United States varies from 2 to 3 miles between stations in highly
developed areas to 75 miles in some of the Western States. It is
general practice for the Cartographic Section to utilize all estab-
lished horizontal ground-control stations immediately adjacent to
an area that is being mapped, because they add to the accuracy
and simplify the preparation of the soil map.
With the radial triangulation method, the distribution of ground-
control stations ideal for accuracy is one in approximately every
16 square miles of area, plus points in the near vicinity of all map
corners, and points in every 3 to 5 photographs along the bound-
aries of the area. Unfortunately, these conditions seldom exist.
Either too few points have been established or they are not dis-
tributed proportionately. Thus, in most survey areas it is neces-
sary to use each control station. Where sufficient ground control
is lacking within the area, it is necessary to use control stations
up to 5 miles beyond its border. Aerial photographs are needed
for the station and for the intervening area. Established ground
control is so sparse in some mapping areas that it is necessary to
use surveys of railroads, highways, and utility companies and the
General Land Office. These, of course, provide a lower degree of
accuracy than proper control stations.
Description of control stations.-Horizontal ground-control sta-
tions fall into three general classes: (1) Monuments, (2) land-
marks, and (3) road and fence intersections.
The first is most important. Monuments are permanently estab-
lished by geodetic triangulation and traverse with a high degree
of accuracy. The exact point located is marked by a concrete
block, galvanized pipe, or cut stone with a bronze station marker
imbedded in the top. Some of the older stations are marked by
triangles or crosses cut into stone monuments or on natural rock
outcrops. Many of these have bronze markers cemented in drill
holes in the rock. Most of these stations are described at length
so that they may be readily recovered. The descriptions include
distances and directions from two or three towns, a route de-
scription to the station site from some town, ownership of the
property on which the station is located, distances from nearby
objects to the station, and angles and distances to reference, wit-
ness, and azimuth marks.
U. S. DEPT. AGRICULTURE HANDBOOK 18
Landmarks used as ground-control stations include church
spires, smokestacks, water towers, air beacons, lighthouses, flag-
staffs, and sharp mountain peaks. Their positions are obtained
through triangulation and they usually have a high order of ac-
curacy. Short descriptions are available, including date of their
Third, and of a lower order of accuracy, are such ground
points as the intersections of the center lines of roads, intersec-
tions of the projections of fence lines with the center lines of
roads, railroad intersections, road and railroad crossings, and
intersections of roads or railroads with section lines. These
stations are obtained along the route of a transit traverse, and
their position and a short description are available from the
establishing agency. They are of relatively lower accuracy than
others because they are not marked in any way on the ground
and the recovery of the precise point established by the traverse
Methods of locating and identifying stations.-With the initia-
tion of a soil survey in an area, the Cartographic Section of the
Division of Soil Survey obtains positions and descriptions of all
established ground-control stations that exist in and adjacent to
the area. The stations are then plotted on some type of existing
map, and aerial photographs are obtained that will completely
cover not only the area of the soil map, but also adjacent ground-
control stations that need to be used for control purposes.
Attempts are made in the Cartographic Section to identify each
control station on a photograph. This is a very exacting task be-
cause the basic construction of the compilation depends on all
points being identified precisely. A large percentage of points are
identified in the office with certainty. Those in doubt or which
cannot be identified are then referred to the soil scientist for
recovery and identification in the field.
The exactness and care required of the soil scientist in the re-
covery and identification of ground control on aerial photographs
cannot be overemphasized. The misidentification of a control sta-
tion on an aerial photograph, if not detected in the radial assembly
of the photographs or in the compilation, will cause distortion in
the scale of the base and displacement of all planimetry and
soil boundaries in the area governed by the station. Even when
the misidentification is found, the station cannot be used, which
weakens the accuracy of the radial triangulation or of the sec-
ondary control points. The field party should use every care pos-
sible when requested to recover and identify ground-control sta-
tions on photographs.
In requesting the information, the cartographers furnish the
party chief with the photograph covering the general area of the
station and with the approximate location of the station indi-
cated on the face of the photograph by a red triangle made with
a grease crayon. A copy of the description of the station is at-
tached to the photograph-
SOIL SURVEY MANUAL
The landmark or intersection class of station is not usually
described at length because it is easily recovered. Perhaps the
major factor in recovery and identification is for the soil scientist
to be positive of the identical point described in the original con-
trol survey. Some intersection stations such as water tanks,
church steeples, and airway beacons are moved, and consequently
their new location on the ground would not be represented by the
old survey position. The date of the survey in the description of
each such station enables the soil scientist to find out from local
residents whether or not the station has been moved since the
control survey established its position. After the soil scientist is
positive of the recovery of the station, he identifies it on the
photograph furnished him for that purpose. A small penciled
circle, preferably in red crayon, about one-fourth inch in diam-
eter, should be drawn around the station on the face of the photo-
graph. On the reverse side, a slightly larger concentric circle
should be placed directly opposite the first one and the name
of the station written nearby in medium-hard black pencil. Ink
should not be used on the face of the photographs used for control
identification. It is not necessary for the soil scientist to locate the
point on the photograph by pricking it; the exact location will be
determined later in the office with the aid of a stereoscope and
pricked with a fine needle. If the field man judges that the cartog-
rapher will have difficulty in pricking the point stereoscopically,
he should include a sketch on the back of the photograph showing
the ground detail immediately around the station.
Monumented stations present more of a problem in field identi-
fication than either of the two previous types, because many can-
not be accurately identified on the photograph even with the aid of
a stereoscope. The monumented stations fall into three classes:
First are those stations that may be accurately pricked with a
fine needle when viewed through a stereoscope or magnifying
glass. The surveyor should circle the pricked point on the back of
the photograph with a medium-hard pencil, write the name of the
station, and, if necessary for clarity, make a small sketch of the
ground and objects immediately adjacent to the station. He may
give a short note on the accuracy of the identification.
Second are those stations that cannot be identified directly be-
cause they are located either in open areas nearly free of detail
or in sparsely wooded areas. With these, the field scientist should
obtain measurements from identifiable objects, such as roads,
fences, buildings, and small trees in the vicinity of the station.
These tie points should be as nearly at right angles to each other
from the station as possible, for this will provide the most accurate
position of the station when it is plotted on the photograph. Three
tie points are sufficient and they may be as much as four or fiv,2
hundred feet from the station. The surveyor should be cautious in
his selection of tie points, for fences and buildings may be moved
or rebuilt, roads may be changed, and small trees may have grown
since the photographs were taken. Usually the soil scientist can
tell whether or not a tie point existed at the time the photographs
92 U. S. DEPT. AGRICULTURE HANDBOOK 18
were taken by referring to the date in the upper left-hand corner.
The tie points should then be pricked with a needle, and a small
red crayon circle placed around each. On the back of the photo-
graph, opposite the area of the tie points, a sketch should be made,
in black pencil, showing the general positions of the tie points,
the station, other pertinent detail, and the measured distances
from the station to each tie point. The name of the station should
be written and a north arrow for direction included. Although
the sketch does not need to be drawn to scale, it should be care-
fully done, because from it the Cartographic Section will identify
the true position of the station.
Third are monumented stations established in heavily wooded
areas where no tie points are available and the station cannot
be identified directly. With these, the soil scientist is limited to
recovering the station on the ground from the description fur-
nished him and by making a careful study of the area on the
photographs with a stereoscope. When he is satisfied that his
identification is the best he can do, he should prick the location,
circle it on the front and back of the photograph, and otherwise
handle like the first group.
The designating characters stamped into the bronze station
markers should agree with the description of the station. The
United States Coast and Geodetic Survey generally places two or
three reference markers in the near vicinity of the station. These
markers are stamped with an arrow pointing toward the station,
and the station itself is stamped with a triangle. Along some
traverses, monumented stations are set in pairs, usually over
500 feet apart, and the soil scientist should take care that he does
not identify the wrong station.
In the examination of photographs in the field for control
purposes, a stereoscope should be used whenever possible. If one
is not available, then a magnifying glass should be used for
pricking all points.
USE OF AERIAL MOSAICS AND PHOTOMAPS
Aerial mosaics and photomaps are generally used in soil sur-
veying much like individual aerial photographs, except they
cannot be used for purposes requiring stereoscopic vision. If the
mosaic or photomap is uncontrolled, its use will parallel that of
the photographs. If it is well controlled, it can be used much like
a well-constructed planimetric map. Since mosaics and photomaps
cannot be studied stereoscopically, more care is needed to locate
accurately drains and other features in densely shadowed areas,
and thereby avoid errors in plotting soil boundaries. Isolated
buildings and trails may be overlooked or inaccurately identified.
A set of stereoscopic photographs may be used, however, along
with a mosaic. These can be retained in the field office, studied
before going into the field, and the necessary interpretations and
delineations made. These interpretations can be transferred to
the mosaic before taking it into the field. In transferring any
changes in cultural features to the mosaic, one must be sure to
SOIL SURVEY MANUAL
note them in a bright contrasting color and be certain that they
are placed on the set of sheets that carries the plotting of
Matching on controlled mosaics and photomaps is much simpler
than on individual photographs. Mosaics and photomaps are
usually constructed to be reproduced in quadrangles or similar
sheet forms. These sheets are usually bounded by latitude and
longitude lines and will fit together with no overlap. Their outer
grid line serves as a matching boundary, and the detail can be
matched by abutting the adjoining sheets. If the mosaic or
photomap has an overlap area, it is usually very narrow, and
grid lines that can be used for matching will usually appear on
the inward side of the overlap area.
Since mosaics and photomap sheets normally cover a larger
area than individual aerial photographs, fewer sheets are re-
quired to cover the survey area. This greatly reduces the number
of match lines between sheets and the time required to transfer
and to match boundaries.
Inking on mosaics and photomaps is done like it is on photo-
graphs. Overlays over a photomap or mosaic are handled like
overlays over the individual aerial photographs. Since mosaic and
photomap sheets are large, overlay material must possess good
The use of poor, uncontrolled mosaics can frequently lead to
serious difficulties in field mapping and later construction of the
map-difficulties that may result in a poor map and increased
USE OF TOPOGRAPHIC AND PLANIMETRIC MAPS
When using topographic and planimetric maps obtained from
reliable sources and constructed by precise methods, one can
assume that the cultural and physical details have been properly
plotted and classified, that place names are correct, that projec-
tions and grids are accurately laid out, and that the map conforms
generally to high accuracy standards.
Although more accurate than aerial photography, topographic
and planimetric maps lack the minute ground detail appearing
on photographs that is so helpful in soil mapping. Also such
maps are commonly on too small a scale for field mapping. As
topographic and planimetric maps are bounded by grid lines and
are so precisely constructed that adjoining sheets match, there
is little or no difficulty in joining them.
Inking on the topographic or planimetric map is done with a
good grade of waterproof drawing ink. As cultural and drainage
features are already printed on the map, inking involves only
the soil boundaries, symbols, and changes in features. Such
changes are plotted by plane-table methods and inked on the
maps in a bright contrasting color such as red or carmine. Old
features that have been abandoned or no longer exist are crossed
out in red ink. It is helpful if these corrections and changes are
made on the set of maps on which the soil data are plotted.