provide an excellent technical source of data. Because these maps are produced for a limited function and area,
they may be difficult to locate, obtain, and reproduce.
2. Remotely Sensed Imagery. A remotely sensed image is any image of the earth's surface that has been
recorded by a device not in physical or intimate contact with the object or phenomenon under study. These
images can provide a large portion of the information required to analyze geologic data and construct a geologic
overlay. Individual rock formations and structures, such as faults and folds, are often identifiable on remotely
sensed imagery. The accuracy and detail that can be discerned from the image depend largely on the season and
scale of the imagery as well as on the skill and knowledge of the analyst. There are two broad categories of
imagery frequently used by terrain analysts in evaluating geologic conditions-aerial photography and
a. Aerial Photography. Aerial photographs are images of the earth's surface that have been recorded using
airborne sensors which detect the reflectance of light in the visible or near-visible spectrum. The images may be
either black and white (panchromatic) or colored. One especially useful type of aerial photograph is known as a
stereopair (also stereoscopic pair or stereogram). A stereopair is made up of two photographs taken of adjacent
areas in such a way that a portion appears on both photographs. For optimum stereoscopic (or three-dimensional)
viewing, the photography should have a 60 percent overlap along the flight line and a 30 percent overlap along
two adjacent flight lines. The best aerial photographs available are 9-inch by 9-inch prints with a scale of
1:20,000 or larger.
b. Multispectral Imagery. Multispectral imagery is imagery that has been obtained simultaneously in a
number of discrete bands in the electromagnetic spectrum (not just in the visible or infrared part of the spectrum,
as in aerial photography). LANDSAT and SPOT are two examples of multispectral imagery that can be used as
an excellent source of information supporting regional analysis. The most common scale for this type of imagery
is 1:250,000, but almost any scale can be obtained. It has been found that electromagnetic wavelengths of 1.55 to
1.75 microns and 2.00 to 2.35 microns (midinfrared range) are best for geologic interpretation. These
wavelengths correspond to LANDSAT Bands 5 and 7. A course on multispectral imagery and interpretation will
significantly improve the analyst's ability to request and interpret this imagery.
3. Geologic/Geographic Literature. This source of information is nearly unlimited in quantity, scope of subject
matter, and coverage of geographic regions. The most useful literature contains information about a specific
geographic area or a specific geologic feature and provides an understanding of the region physiographic
divisions and major topographic landforms. Unfortunately, these sources are often too general to be of any real
value to the terrain analyst. For this reason, area-specific literature should be reviewed by the terrain analyst as
background source material only.
PART B - ROCK TYPE IDENTIFICATION
A rock is an aggregate of one or more minerals. (A mineral is a naturally occurring, inorganic compound that has
a definite chemical composition.) Based on the principal mode of formation, rocks are grouped into three broad
categories: igneous, sedimentary, and metamorphic.