Various sources of information are available. Published information and previous soil analyses are sources you may secure without field exploration. Time sources are used mostly to locate, within a large general area, small areas that you may want to investigate further. For final site selection, actual field investigations must be made. Published information sources include engineer intelligence reports, geologic and topographic maps and reports, agricultural soil maps and reports, and air photographs.

Intelligence reports that include maps and studies of soil conditions are usually available for areas in which military operations have been planned. Among the most comprehensive of these are the Terrain Intelligence Folios prepared by the Intelligence Branch of the U.S Army Corps of Engineers, in cooperation with the U.S. Geological Survey.

Geologic maps and brief descriptions of regions or quadrangles are published in the folios of the U.S. Geological Survey. Generally, the smallest rock unit mapped is a formation; geologic maps indicate the extent of formations by means of letter symbols, color, or symbolic patterns. Letter symbols on the map indicate the location of sand and gravel pits; sometimes the back of the map sheet has a brief discussion entitled "Mineral Resources," that describes the location of construction materials.

Ordinary topographic maps may be of some use in estimating soil conditions and can be used in conjunction with geologic maps. Inspection of the drainage pattern (as indicated by contour lines) can provide clues as to the nature of rocks, depth of weathering, soil, and drainage.

Agricultural soils maps and reports are available for many of the developed agricultural areas of the world. These studies are usually concerned primarily with surface soils to a depth of about 6 feet. Information given includes topography, drainage, vegetation, temperature, rainfall, water sources, and rock location. Soils are usually classified according to texture, color, structure, chemical and physical composition, and morphology (topographic features produced by erosion).

The use of aerial photographs to show and identify soils is based upon your ability to recognize typical patterns formed under similar conditions. An example might be soil profile and weathering. Principal elements that can be identified on a photograph and that provide a trained observer with clues to the identification of soils are landform, slopes, drainage patterns, erosional characteristics, soil color or "tone," vegetation, and land use.

The form or cofiguration of the land in different types of deposits is definitely characteristic and can be identified on aerial photographs; for example, in desert areas, characteristic dune shapes indicate areas covered by sand subject to movement by wind.

Prevailing ground slopes are clues as to the texture of the soil. Steep slopes are characteristic of granular materials, while relatively flat and smoothly rounded slopes may indicate more plastic soils.

The absence of surface drainage or a very simple drainage pattern often indicates pervious soil. A highly integrated drainage pattern often indicates impervious soils that are plastic and usually lose strength when wet. Drainage patterns tend to reflect underlying rock structure.

The pattern of erosion often provides clues as to the character of the soil. For instance, the cross section or shape of a gully is controlled mainly by the cohesiveness of the soil. Each abrupt change in grade, direction, or cross section indicates a change in the soil profile or rock layers. Short, V-shaped gullies with steep gradients are typical of noncohesive soils; U-shaped gullies with steep gradients indicate deep, uniform silt deposits. Cohesive soils generally develop round, saucer-shaped gullies.

The color of soil is shown on aerial photographs by shades of gray; they range from almost white to almost black. Soft, light colors or tones generally indicate pervious, well-drained soils. Large, flat areas of sand are frequently indicated by uniform, light gray color tones, a flat appearance, and a lack of conformity; this indicates a natural surface drainage. Clays and organic soils frequently appear as dark gray to black areas. In general, a sharp change in color tones represents a change in soil texture.

The character of the vegetation may reflect the surface soil type; however, its significance is often difficult to interpret because of the effects of climate and other factors. To those with local experience, both cultivated and natural vegetation cover are good indicators of soil type.

Knowing the use to which agricultural land is put is often helpful in soil identification. For example, orchards require well-draining soils; therefore, the presence of an orchard implies a sandy soil.