reports. Because maps and soil reports contain written information concerning various soil types, their
interpretations are fairly straightforward. The interpretation of aerial photographs, on the other hand, is generally
more difficult. Therefore, the procedures that should be followed in analyzing aerial photography are discussed in
the following paragraphs.
1. Analysis of Tone. The various objects that comprise a geographic region exhibit their own unique colors and
textures, resulting in characteristic light reflectances. For example, coarse, well-drained materials or materials in
arid regions tend to be very light (almost white) in tone, whereas fine-grained materials with poor drainage tend to
be represented by gray tones. Dark gray to black tones are associated with fine-grained materials containing large
amounts of organics and/or very high moisture contents.
Uniformity of tone is an important aid to the photo interpreter. Uniform soil conditions usually exhibit uniform
tone, while nonuniform soil conditions normally result in mottled or mixed tones.
Although photographic tone is an important consideration, it should be noted that tone is affected by several
factors, such as the relationship between the sun, the object, and the camera; the type of film used; the length of
exposure; and the procedures allowed during processing. Therefore, it is difficult to develop standard values for
tonal variations. This allows for the subjective interpretation of aerial photographs. In fact, the same tones and
patterns are frequently perceived and interpreted differently by different analysts. Obviously, care should be
taken in using tone as a photo-interpretive element.
2. Analysis of the Effects of Erosion. The erosional patterns evident on an aerial photograph are the result of
the actions of wind, water, and/or ice. As previously mentioned, gullies are erosional features that are particularly
useful in the determination of soil textures. A discussion of the way in which soil types may be determined
through interpretation of gully profiles is given in Lesson 2.B.1.a.(1)(a) page 2-4. In order for gully analysis to be
useful as a photo-interpretive key, the scale of the photograph must be sufficiently large-generally 1:20,000 or
larger. In areas where soil cover is easily observed, such as in arid or semiarid regions, interpretation tends to be
more reliable.
3. Analysis of Drainage. The drainage patterns and textures present on aerial photographs are possible
indicators of the origin, composition, and type of underlying soil, as described in Lesson 2.B.1.a.(1)(b) page 2-4.
Drainage is often considered to be the single most important photo-interpretive feature because of the ease with
which stream courses may be detected and delineated on aerial photographs.
4. Analysis of Topography. Through the use of aerial photography, terrain analysts may determine the relative
elevations of various areas of the topographic surface, thereby gaining information concerning the types of
materials present in those areas. As discussed in Lesson 1, ridges existing in humid regions underlain by
sedimentary rocks are normally composed of sandstone, whereas the valleys are often made up of limestone or
shale. As a result, fairly coarse-grained soils, which form from the weathering of sandstone, are generally
encountered on or near ridges, while clays or silts are usually found in valleys where limestone or shale has
undergone weathering.
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EN5341
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