USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 725

24. Fisk, H. N. Geological investigation of the alluvial valley of the lower Mississippi River. Con­ducted for the Mississippi River Commission, Vicksburg, Mississippi, December 1944, 78 pp.,

·80 figures (51 photographs), 33 plates.25. Fisk, H. N., ibid., p. 1.26. Fisk, H. N., ibid., p. 1.27. Howe, H. V., and Moresi, C. 1<., Geology of Lafayette and St. Martin Parishes, Louisiana

Geological Survey Bulletin No.3, July 1933, p. 18.28. Fi~k, H. N., op. cit., p. 42.29.. Zernitz, Emile R. Drainage patterns and their significance, Journal of Geology, Vol. 40, 1932,

pp. 498-521.30. Melton, F. A., op. cit.31. Russell, R. J. Louisiana stream patterns. Bull. Amer. Assoc. Petrol. Geol., Vol. 23, o. 8,

August 1939, pp. 1199-1227.32. Fisk, H. N., op. cit., p. 21.33. De Blieux, Charles, Photogeology ill Gulf Coast exploration. Bull. A mer. Assoc. Petrol. Geol.

Vol. 33, No.7, July 1949, pp. 1251-1259.34. Howe, H. V., and Moresi, C. K., op. cit., pp. 75-76.35. Barton, Donald C. Surface fracture system of South Texas, Bull. Amer. Assoc. Petrol. Geol.

Vol. 17, No. 10, October 1933, pp. 1194-1212.36. Fisk, H. N., op. cit., p. 66.37. Dickerson, R. E. Trends of ocean currents across the Florida Everglades. Geographical Review,

Vol. 32, 1942, pp. 135-139.38. Stokes, G. A. The aerial photograph: a key to the cultural landscape. Journal of Geography,

Vol. 49, No. I, January, 1950, pp. 32-40.

USE OF AERIAL PHOTOGRAPHY FORINDIANA LAND STUDIES

T. M. Bushnell, In Charge of Soil Survey, Agricultural ExperimentStation, Purdue University, Lafayette, Indiana

T HIS paper will briefly tell how aerial photography came to be used by theIndiana Soil Survey, and some of the ensuing applications, and interrela­

tions with other projects.Several Soil Surveyors, in the Aviation Section of the U. S. Army Signal

Corps in 1918, perceived that aerial observation afforded the precise viewpointwhich they had assumed in making soil maps, and that aerial photos recordedthe scene with remarkable detail and fidelity. Such vertical observation potentlysupplemented the surface and subsurface information by which Soil Surveyorsdescribe and interpret landscapes in terms of soils.

Although the values and desirability of aerial photos in soil studies were atonce apparent to field men, progress was slowed down by problems of costs,techniques and administrative approval. .

In 1925 $23 per square mile was quoted for aerial photos of a county whenthe cost of Soil Survey field work was about $5 per square mile in Indiana. Thatprice precluded use of pictures.

In 1926 Dr. Mark Baldwin and the writer consulted Capt. Goddard (nowColonel) at Chanute Field about aerial photos for soils mapping, and subse­quently received some prints covering areas near Lafayette. These picturesallowed some early field trials in their use.

In 1927 prints were obtained of a strip taken for highway location in Martinand Lawrence counties, Ind. Several state and Federal soil surveyors partici­pated in experimental mapping with these aerial photos. The current ideas ofthat time and some hopes for the future were given before the Indiana Acaciemyof Science3 in Dec. 1927.

726 PHOTOGRAMMETRIC ENGINEERING

In 1928, some optimistic plans for photographing Rush County throughcooperation of the U. S. Army Air Service,4 were made possible by the farsig~ted

approval of Director G. I. Christie of the Purdue University Agricultural Ex­periment Station. That plan was thwarted by a series of circumstances.

Between 1925 and 1929 the cost of soil mapping by plane-table traversegreatly increased due to use of larger scales, higher standards of accuracy andother items of expense. In the meantime the availability of aerial photographicservices increased, and the unit costs came down, to the point where it wasfeasible for soil survey to obtain aerial photos from commercial sources.

In 1929 Jennings County,S Indiana was covered by contract at a cost of $5.25per square mile. That area of 360 square mires was flown in 3 clear days-some­thing rare in November.

There was little local information or experience available. at that time re­garding the effective use of such pictures as a base for soil mapping. However,the training of men in the use and interpretation of the pictures was surprisinglyeasy. Soil surveyors were able to recognize many familiar features of the land­scapes in the pictures. In the case of uncertainty they checked back and forthfrom picture to land until they could identify the questionable features on thephotographs.

The technique6 employed was to draw the map on some transparent materialover the pictures. Overlay material at that time was rather unsatisfactory be­cause of being too opaque, too flimsy, too smooth to take lines, and too subjectto shrinkage. Some of that first film even decomposed-fortunately not untilafter the data had been transferred. Two general classes of overlay were found.One, which was clear, was easy to see through but required use of ink in thefield. Etched surface materials were harder to see through but would take pencilmarks. The pencilling however had to be done over later on in order to makegood photographic copies.

The use of overlay automatically separated the map data when the sheet

FIG. 1. A Soil Surveyor easily recognizes the Fairmount soil in Indiana by the pattern ofcontour fields. It follows (1) the local relief is about 200-400 feet (2) the bed rocks are Ordovicianlimestones and shales, (3) the small bottons have brown, sweet soils, (4) the ridge road probablyis on silt loam with heavier, yellowish, acid subsoils.

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 727

FIG. 2. The patterns of featureless white fields, and others with straight parallel stripes aretypical of old flat soils with acid clay-pans. The stripes are due to "lands" with dead furrows to

•give surface drainage. The pattern of ·"sinks" near the stream valley indicates limestone sub­stratum, but the combination of all these features is characteristic of the Illinoian till plains ofsoutheastern Indiana-to the exclusion of comparable plains in the southwest, and certain residualsoils. ,

FIG. 3. The spotted pattern indicates there is a dry gravel substratum. The darker tones onthe left show higher humus, grassland soil in contrast to the lighter colored timbered land to theright of the railroad. The very dark areas in the upper right are of wet or vegetated bottom lands.It is probable that the soils in the dark spots and streaks on the left of the picture are really notas darkly colored as they seem. They are slightly lower, deeper and finer textured-butextra moistureand vegetation probably account for the extra dark shades. Such land patterns are characteristic ofglaciofluvial valley trains.

728 PHOTOGRAMMETRIC ENGINEERING

FIG. 4. The strips parallel to the old oxbows of the river are probably all brown fresh alluviumwith the different shades in the picture related to texture and moisture content. Such soil is surelyneutral, but the smoother pattern on the right may be of somewhat acid, older soils with definitehorizons.

was lifted from the picture over which it was drawn. Variations in scale and tiltof the photos made necessary the rectification of the map data so that the indi­vidual parts could be fitted together on assembly sheets. These sheets were oftransparent material on which control points were plotted, by which each smallarea of mapping from individual pictures could be put in its proper place.

The method and apparatus for rectification has been described elsewhere,7but briefly included fitting the map image from the overlay to the control points,then making a photographic positive with white lines on black background.These rectified prints were then fitted under, and traced upon transparent as­sembly sheets, which usually covered an area of one township. Each assemblysheet was originally prepared by tracing control points from one large base map;this insured the sheets fitting together perfectly.

The large base map, on heavy drawing paper, was a polyconic projection forthe county, with geodetic points plotted upon it. In almost all Indiana counties,such precise points, which also could be located on the aerial photos, are veryscarce: A network of other control points had to be developed to fix locations ofall photos. Many methods were tried-planetable traverse, radial line, straightline and section line control, General Land Office data, railway and highwayalignments, and a few cases of primary traverse and topo maps to be found inIndiana. These data made possible the construction of Indiana soil maps fromaerial photos, but in many counties there were discrepancies in control and somedistortion of the resulting soil map. More recently the Federal SoiJ Survey draft­ing office has prepared base maps, with control points located by the slottedtemplet method, which are practically perfect for assembly of the soil data.

Economy was a watchword in the early use of aerial photos in Soil Survey.The mappers were expected to cover the land with sufficiently greater speed torepay the price of pictures and to make work done with photos cost no more perunit area than the plane-table method of mapping. In 1933 some administratorsquestioned whether Soil Survey could afford to use aerial photos. An analysis of

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 729

FIG. 5. The combination of straight roads and landlines, dredged ditches, and soft patternsof light and dark soils, with scattered trees and brush are characteristic of the lake plains of north­western Indiana. All this carries the implication of fine sandy texture, light colored crescent dunesand both acid and "sweet" soils with black and pin oaks in the woods.

FIG. 6. Several distinct soil areas are obvious. From the photo without checking the land theauthor "sees" (1) upper left-oval area of relatively dark alluvium-doubtless sweet and fertile.(2) Upper right- soft, mottled pattern of sandy, wind blown soils. (3) Lower right-light coloredsilt loam with calcareous clay subsoil on back-water terrace. (4) Light colored strip just above theriver--doubtless acid alluvium. (5) Lower left-timbered hills-sandstone. These "interpretations"are based not only on this photo, but also upon knowledge of the general geography and observa­tion of the soils listed within a few miles of this area.

730 PHOTOGRAMMETRIC ENGINEERING

FIGS. "7 & 8. These pictures are of the same area in dry September and wet April. 'Many ofthe white streaks in the April picture are due to dryer soil above tile lines while equally light, butwet soil, between the tile photographs darker.

costs of field work for 8 Indiana counties done without photos, by plane-tablemethods, showed $14.80 per square mile. Eight counties made with picturesaveraged about $13.80 per square mile, while the cost per square mile for 4counties made with some inexpensive photos was $11. This comparison on costalone, did not consider the more important point that soil maps made withpictures were far more detailed and reliable in all ways than the plane-tablemaps.

In 1930, Knox, Washington and Vermillion counties were flown by com­mercial contract, and the soils were mapped in the same year. Vermillion Countywas the firstZ county published, although Jennings County 17 was the first areamapped with aerial photos as a standard device of the survey technique.

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 731

FIGS. 9 & 10. The main contrast between these two pictures of the same ar~a is due to reflec­tion of sunlight'from a lake in one of them-not in the other. The dark marsh or muck lands areequally dark in both pictures.

In 1931, a contact with Lt. Howard H. Maxwell of the 113th ObservationSquadron, resulted in an agreement whereby Purdue paid certain expenses ofaerial photography in selected counties, which National Guard personnel flew aspractice missions. In that way the officers and men got extra experience in theirwork, and Purdue got sets of pictures at about $1.00 per square mile. TheFederal Soil Survey cooperated by purchasing sets of prints at about half thecost of the jobs. A number of obliques and vertical pictures of Purdue land werealso made by the Guard. This arrangement continued until Purdue had picturesof 21 counties. Commercial interests then objected to this "competition." Subse­quently Purdue got prints of the pictures taken for A.A.A. at still lower unitarea costs than was possible from the National Guard.

For several years there were many efforts3 to develop a coordinated program

732 PBOTOGRAMMETRIC ENGINEERING

FIGS. 11 & 12. These pictures cover approximately the same area. They were taken 8 yearsapart, some points almost are identical and other patterns are enough different to definitely affectsoil interpretation and mapping. Such patterns could be one of several different catenas, but thelocation of this tract along the Muscatatuck River between Illinoia till and residual hills places itas the McGary Montgomery soils-of Wisconsin age-leached of lime to a depth of about 3 feetand entirely free from stones.

of aerial photography in Indiana, both for the sake of broader support with aresulting sharing of expense and for the benefits which could accrue to all of theagencies which would use the pictures. This cooperation was never attainedalthough several public projects got aerials for their own individual purposes.Topographic survey and conservation survey programs obtained some picturesseparately. About 1940 the A.A.A. program of aerial photography became apractical substitute for proposed coordination, by making pictures available toboth public and private groups or individuals.

In the first years of its program, A.A.A. supplemented its own sets of pictures

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 733

by enlargements made from aerial negatives which the Indiana Soil Surveyalready had obtained of certain counties. The special apparatus and procedureswhereby about 10,000 rectified enlargements were made for A.A.A have beenpreviously described in this journal. 7 ,9

The role of aerial photos has changed with changing times, considerationsand personnel. It is now taken for granted that'aerial pictures are the best, oronly, way to make certain studies. Their values for accuracy and varied infor­mation are recognized and costs have decreased relative to other expenses ofsurveys. Some survey work has become so costly that pictures are very minoritems in the budget.

This changed economic relation of aerial photos has had some effects on soilstudies. It is no longer considered advisable to preserve <J. set of pictures. Map­ping directly upon prints is common practice. That method also requires inkingof pencilled lines, and apparently less convenient or economical procedures forrectification and' assembly of soil maps, than was possible with the overlaymethod.

Another consideration in techniques is the final use of the data. In the Indi­ana Soil Survey the objective was preparation of copy for a published map onsmaller scales. The checking, harmonizing of individual mapping, reduction, andimprovement of drafting was accomplished in the assembly process. In someprojects the objective is not publication, but enlargement of picture and map­ping data, for use of planners and farmers. Only a few copies are required. Thisplan calls for larger and more perfect original soil maps drawn directly onpictures.

There have been several very interesting and important ramifications in soiland land work at least partly due to availability of aerial photos.

For years, soil survey reports have been expected to tell how each type ofsoil is used and something of its adaptations, limitations, and value. About theonly basis for opinion was incidental observations of field parties during thecourse of field work. In 1935, the field party of Bartholomew County, Indiana,attacked that question by mapping "cover" on a duplicate picture (or overlay)while mapping soils. Very little extra time or effort was spent in mapping cover,beyond that needed to map the soils. Considerable time was required to tabulateand interpret those data, but fortunately clerical help was available throughfunds for National Youth Administration projects. All members of the SoilSurvey staff were involved in field work and office consideration of cover data,but much of the clerical work was under the supervision of B. A. Krantz16 whowrote a Master's thesis entitled "Use Adaptation as Indicated by Present LandUse in Indiana." Methods were worked out and many tables and graphs pre­pared to show uses, factors affecting uses, an adaptability index etc. Such studieshave been continued. with land use being mapped in great detail in all IndianaSoil Surveys. The data have been tabulated in only part of these counties, butthey make possible the present land use map for soil regions as shown in theStory of Indiana Soils. 8

These land use-soil studies, so greatly facilitated by aerial photos, have beenapplied in a number of ways. For instance a large body of material has beenworked up in connection with land valuation, although not released in publishedform. These studies have correlated soil types, and soils grouping, according toselected characteristics and productivity ratings, population pressure, locations,sizes of farms, management etc. with census values, assessed valuations, salesprices, appraisals by loan agencies etc. Some portions of the Indiana taxmanuaJ13were based in part on land data so collected and interpreted by the Indiana SoilSurvey.

734 PHOTOGRAMMETRIC ENGINEERING

FIGS. 13 & 14. These pictures of the same land in different seasons show how patterns canchange. In the lower left are two fields. One field is whi te and featureless, while the other field showssome informative details in picture 13. These two fields have just the opposite appearance inpicture 14.

The benefits of experience with soils and aerial photos were passed on to someextent by the Indiana Soil Survey to the Purdue Joint Highway Research Proj­ect, about 1940-43 through consultations in the office, field trips and loan ofsome pictures. The Joint Highway Research group has reported its findings in anumber of publications1 ,IO,Il,12,14,18,19 and have extended their studies over mostof the U. S., Alaska and part of Canada.

As previously indicated, the technique of photo interpretation used by SoilSurveyors is basically one of recognizing familiar features. There is no reasonwhy this project should try to depend on pictures alone. In fact there are manyimportant aspects of .soils which pictures, at best, cannot show. All serious soilsurveys primarily rely on direct observation of the surface pattern, and below

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 735

the surface profiles. Pictures giv.e locations, outlines of areas and other featureswhich are a great help in judging what to expect.

On the other hand the pictures may be used for "office mapping" of areas,either extensively or in detail, with considerable assurance about the dominantsoil conditions. Many inquiries about farms may be answered in part by thismethod.

If the soils man is entirely unacquainted with the soils, vegetation, use pat­terns etc., in some unfamiliar part of the world; his interpretations of picturesare likely to be much more limited in detail and reliability.

In general soil survey groups have done comparatively little in formal codifi­cation of the features they use in photo interpretation. Unconsciously they dis­tinguish photo details they relate to cover, from those of the earth's surface, andalso note the appearance which may indicate something about the topographyor the subterranean conditions. These details are known largely by experiencewith the changing aspects of landscapes under different uses, with the seasons,with the time of day, with wet or dry conditions or other factors which modifythe picture patterns. A man may say "that is a certain soil because it looks likeit" with no more critical analysis than when he recognizes the picture of afriend. It is easy to point out conspicuous and important differences, such aslight and dark soils in bare fields, or characteristic shapes and locations of soilareas such as bottoms, or the "sinks" indicative of limestone formations under­ground, and somewhat similar pits showing occurrence of dry gravel beds underoutwash plains. Representative photo patterns are shown in the Story of IndianaSoils. 8 Innumerable "signs" are hard to put into words, although apparent tothe trained eye of experienced students of land. This discussion will not evenattempt to list all the criteria.

It is important to recognize that a Soil Surveyor does not take a restrictedview of his field, but considers "the soil" as a summation of almost all important

FIG. 15. The dark colored streaks show dark soils on shallow depressions through light coloredalmost level soils. Undoubtedly there are similar contrasts in the several fields which are very darkall over, due to heavy growth of legumes. Several different catenas of soils might have such patternsbut with the additional information that this area occurs on the "early" Wisconsin till, the soilsman may interpret the light soil as Fincastle silt loam with unleached calcareous till at 4 to 6 feetbelow the surface.

736 PHOTOGRAMMETRIC ENGINEERING

FIGS. 16 & 17. These two pictures cover about the same area of smooth till plain land, formerlytimbered, with light and dark soil pattern typical of the Tipton till plain of central Indiana. Theywere taken several years apart, but the main contrast is due to effects of a preceding rainy period.On Fig. 16, on the right edge and south west corner are several areas which are really dark depres­sional soils but which are covered with a thin film of light, silt washed in from the surroundingtruly light colored rises. Fig. 17 shows the light shade disappeared on cultivation.

factors of any landscape. This is in line with the basic theory of factors of soilformation8,15-namely, that all soils are functions of materials, climate, vegeta­tion, topography or water regime and age. In other words, every soil reflectsthe geological materials and age, the rainfall, temperatures, 'nativ~ vegetation,slopes, water table relationships etc. of each local site. Hence the soils studentcan read soil features directly from aerial photos and from these features in­terpret definite geological, climatic, and ecological relationships. In addition hecan read agricultural, sociological, economic, and historical facts related tohuman occupance.

USE OF AERIAL PHOTOGRAPHY FOR INDIANA LAND STUDIES 737

Other applications of aerial photos in connection with agriculture are varied.Such photos can be used in farm management and farm planning; in agriculturalengineering work, such as drainage and bUIlding of conservation structures;in selecting lands adapted to various uses. New questions constantly arise whichare partially answered by aerial pictures and material prepared with aid ofphotos. .

In spite of extensive evidence of the wide range of 'uses and reliability ofphotos, it should not be concluded that they are infallible. To the contrary thereare many Cases where pictures may be uninformative, confusing or actually mis­leading. One indication of that possibility is the greatly varied appearance ofselected tracts of land in 2 or more pictures covering the same area. The presenceor lack of leaves on trees, the kind and condition of crops, the moisture conditionof the surface, may camouflage or greatly vary the land patterns. Some "blacksoil" in depressions may look very light due to a thin film of silt over the surface,which would disappear on plowing. A good growth of vegetation along draws insloping land may give a pattern resembling dark soils in swales. The use of astereoscope helps to avoid some mistakes. In the case of heavily wooded hills astereoscope will help locate stream valleys which may be hard to distinguish onthe ground and pictures alone. Photo-interpretation needs lists of possible errorsalmost as much as keys to positive signs of land conditions.

The future seems to promise even more universal use of aerial photos in allsorts of applied and research projects. The general-purpose pictures of theA.A.A. are adequate for many uses and may be greatly enlarged before becom­ing too blurred. Also, there are indications of increasing demands for more pre­cise, costly, and special sets of aerial photos, although that need is not so muchin agriculture as in engineering or other fields.

REFERENCES

1. Belcher, J. D., L. E. Gregg, and K. B. Woods. The Formation, Distribution, and EngineeringCharacteristics of Soils. Engineering Bulletin of Purdue University, Research Series No. 87,Vol. 27, No.1, January, 1943.

2..Buckhannan, W. H., and J. S. James. Soil Survey of Vermillion County, Indiana. Soil SurveyReport No. 20, Series 1930. Purdue University. .

3. Bushnell, T. M. Aerial Photography for Indiana. Proc. Ind. Acad., Sci., Vol. 37, 1927 (1938).pp.63-72.

4. Bushnell, T. M. Aerial Photography and Soil Survey. The Amer. Soil Survey Assoc., Bulletin X,pp. 23-28, 1929.

5. Bushnell', T. M. Aerial Photographs of Jennings County. Proc. of Ind. Acad. of Sci., Vol'. 39,1929.

6. Bushnell, T. M. A New Technique in Soil Mapping. The Amer. Soil Survey Assoc., BulletinXIII, pp. 74-81, 1932. .

7. Bushnell, T. M. The Rectification of Enlargements and of Map Data from Aerial Photos bvIndiana Soil Survey. PHOTOGRAMMETRIC ENGINEERING, Vo!. V, No.3, July, August, Sep­tember, 1939. p. 147.

8. Bushnell, T. M. The Story of Indiana Soils. Special Circular 1, June, 1944. Purdue UniversityAgric. Exp. Sta.

9. Coblentz, C. S. Restitution of Aerial Enlargements for Area Determination. PHOTOGRAM­METRIC ENGINEERING, Vol. V, No.3, July, August, September, 1939. p. 138.

10. Frost, R. E. The Use of Aerial Maps in Soil Studies and Location of Borrow Pits. Proc. of the. Kansas Highway Eng. Conference, pp. 58-82, July I, 1946 (Engineering Reprint of the Joint

Highway Research Project, No. 20).11. Frost, R. E. Identification of Granular Materials from Aerial Photographs. Joint Highway Res.

Proj., Proc. of the Thirty-second Annual Purdue Road School, Extension Series No. 61, Vol.30, No.4, pp. 105-136, January, 1946 (Engineering Reprint of the Joint Highway ResearchProject, No. 23). .

12. Frost, R. E., and J. D. Mollard. New Glacial Features Identified by Airphotos in Soil MappingProgram. Proc., Highway Research Board, Vol. 26, pp. 562-578, December, 1946 (EngineeringReprint of the Joint Highway Research Project, No. 29).

738

I

PHOTOGRAMMETRIC ENGINEERING

13. Ind. State Board of Tax Commissioners. Real Estate Appraisal Manual, Nov., 1948.14. Jenkins, D. S., Belcher, D. J., Gregg, L. E., and Woods, K. B. The Origin, Distribution, and

Airphoto Identification of United States Soils. Technical Development Report No. 52, UnitedStates Dept. of Commerce, Civil Aeronautics Admin., Washington, D. c., May, 1946.

15. Jenny, Hans. Factors of Soil Formation. McGraw-Hill Book Company, Inc. New York andLondon, 1941.

16. Krantz, B. A. Correlation of Soil Types with Present Land Use. Rep. from Soil Science Soc.of Amer. Proceedings 1940, Vol. 5, pp. 309-312.

17. Kunkel, D. R. Soil Survey of Jennings County, Indiana. Soil Survey Report No. 40, Series1932. July, 1940.

18. Mollard, J. D. Airphoto Mapping of Montgomery County Soils for Engineering Purposes.Proc. of the Thirty-Third Annual Purdue Road School, Ext. Series No. 63, Vol. 31, No.4,pp. 223-266, July, 1947 (Engineering Reprint of the Joint Highway Research Project, No. 36).

19. Parvis, Mer!e. Development of Drainage Maps from Aerial Photographs. Proc. of the HighwayResearch Board, Vol. 26, pp. 150-163, December, 1946 (Engineering Reprint of the JointHighway Research Project, No. 27).

THE USE OF AIR-PHOTOS FOR SOIL CLASSIFICATIONAND MAPPING IN THE FIELDl

John D. Rourke and Morris E. Austin2

U SE of air-photos as bases in soil survey work has steadily increased sincethey were first used in 1929 by Bushnell and his co-workers. They have

now almost entirely displaced all other types of bases for soil survey work.Indeed, unless a soil map for a particular area is urgently needed, mapping isnot undertaken until air-photos are available. The clarity with which air-photosshow detail of cultural and land features has, in fact, led some to be over­enthusiastic about them, and to suggest that a soil survey can be interpretedlargely from them without field work. Useful as they are as bases for field workin soil mapping, little about soils can be learned from air-photos without carefulstudy on the ground. The photos permit rapid and accurate plotting of soilboundaries, but sound classification and mapping of soils can be achieved onlyby 'detailed field study of the soils themselves, supplemented by laboratorystudy of samples. Examinations of soil profiles are essential. Air-photos showsoil patterns, not soil profiles. To the competent soil scientist who has studiedthe soils of each pattern on the ground, air-photos do suggest the kind of profilespresent.

THE SOIL TYPE

We shall first review briefly what soils are and how they are classified, forthis information is essential to an understanding of the use of air-photos insurveying. Soils may be defined as "the collection of natural bodies occupyingportions of the earth's surface that support plants and that have properties dueto the integrated effect of climate and living matter, acting upon parent ma­terial, as conditioned by relief, over periods of time."

Soil consists of several more or less distinct layers, or horizons; and the cross­section of these horizons from the surface of the ground to the parent materialis known as the soil profile. Soils vary greatly both in degree and kind of profileexpression, depending upon the intensity of the different soil-forming factors,

1 Material for this paper has been drawn from many sources, including published papers, re­ports, books, and the experience of many soil surveyors. Specific literature citations are thereforenot made in the text, but a few selected references listed at the end of the text deal with the subjectmatter of the paper.

2 Soil Scientists, Division of Soil Survey, BPISAE, U. S. Department of Agriculture.