PART A - SOURCES OF INFORMATION
In performing a geologic evaluation, the terrain analyst utilizes several types of available source materials,
including maps, imagery, and literature. In many areas, geologic maps of various scales already exist. If these
maps do exis, the terrain analyst should exhaust all efforts to procure copies for the data base. Other useful
materials required to update existing studies or to construct new studies include current 1:25,000 and 1:50,000
scale topographic maps, aerial photography at scales ranging from 1:20,000 to 1:40,000, land satellite
(LANDSAT) or Systeme Probatoire d' Observation de la Terre (SPOT) imagery, and regional studies of
landforms, geology, and geomorphology. These materials may be obtained from various governmental agencies
(such as the United States Geological Survey, the Defense Mapping Agency, the National Aeronautics and Space
Administration, the United States Soil Conservation Service, the National Forest Service, and state geological
agencies) and private organizations as well as from books and periodicals. It is often necessary to use materials
prepared by foreign governments or companies, such as reports available from numerous mineral and gas
exploration efforts. The adequacy of source materials varies from one area to another; therefore, the analyst
should use as many different sources as possible to achieve maximum accuracy and completeness of the data
base. If the information sources are questionable, the analyst should annotate the uncertain reliability of the data.
In some cases, there may be no source materials readily available, in which case it may be necessary to plan and
request collection support. The collection of geologic information is an ongoing process whereby the analyst
continually updates the data base with more current and increasingly accurate data. Once a sufficient amount of
data has been collected, the analysis process begins with a critical review of the data base materials on hand.
1. Maps. In addition to geologic maps, there are three other basic map types available that can be used to
construct a geologic overlay: topographic maps, surface configuration maps, and landform distribution maps.
a. Topographic Maps. Standard topographic maps displaying elevation and planimetric data are available
at scales ranging from large (1:25,000) to small (1:1,000,000). Maps produced at large scales allow for the
extraction of more information than those produced at smaller scales. However, geologic overlays are normally
produced at scales of 1:50,000, so information derived from maps with scales other than 1:50,000 need to be
reduced or enlarged to be used in conjunction with the overlays. In addition to the use of topographic maps for
the extraction of relief and areal data, the analyst also uses them for the delineation of drainage patterns, which is
a primary interpretation key in the determination of rock type and structure.
b. Surface Configuration Maps. These maps are usually produced at small scales (1:1,000,000 or smaller)
and can be found in virtually any geography text or world atlas. They depict very broad categories of surface
configurations. In general, these maps and associated descriptions will not provide the detail required for specific
landform identification. However, they should be reviewed by the analyst for extraction of data in support of
large-scale country studies and for general familiarization with a specific area of interest.
c. Landform Distribution Maps. Landform distribution maps, often called physiographic maps, are
characterized by their large scales and detailed information; they