Geology Reference
In-Depth Information
EXERCISE
2
Maps, Aerial Photographs,
and Satellite Images
Relief refers to difference in elevation between the tops
of hills and the bottoms of valleys within a given area.
The datum plane is the plane of zero elevation, which
for nearly all topographic maps is mean sea-level.
Elevation is the vertical distance between a given point
and the datum plane.
Height is the difference in elevation between a topo-
graphic feature and its immediate surroundings.
A bench mark is a point of known elevation. It is usually
designated on a map by the letters BM, with the eleva-
tion given to the nearest foot or meter.
Contour lines are lines connecting points of equal eleva-
tion. Contour lines show the vertical or third dimen-
sion on a map and, in addition, the size and shape of
physical features.
Contour interval is the vertical distance between adja-
cent contour lines.
Hachured contour lines are used to indicate closed depres-
sions. They resemble ordinary contour lines except for
the short lines (hachures) on one side, which point
toward the center of the depression.
Scale refers to the ratio of the distance between two
points on the ground and the same two points on the
map. The scale is commonly shown at the center of the
bottom margin of the map sheet and may be expressed
in three ways: (1) a fractional or ratio scale, such as
1:24,000, meaning that one unit of measurement on the
map represents 24,000 of the same units on the
ground; (2) a graphic scale, which is simply a line or
bar that is subdivided to show how many units of dis-
tance on the ground are equivalent to a given distance
on the map; and (3) a verbal scale is a convenient way
of stating the scale, such as "one inch equals one mile."
INTRODUCTION
Maps and aerial photographs are basic tools of earth
scientists. They are scale models of a portion of the
Earth's surface that show details of size, shape, and
spatial relations between features. There are many
different kinds of maps, several of which will be used
in this manual. Therefore, it is very important to have
a basic understanding of how to interpret maps,
aerial photographs and other images.
PART A. TOPOGRAPHIC MAPS
A topographic map is a representation of the configu-
ration of the Earth's surface. In addition to showing
the position, relation, sizes, and shapes of the physical
features of an area, it typically shows cultural features.
Topographic maps are of great value to earth and envi-
ronmental scientists for observation and analysis of
the effects of geologic processes that are constantly
changing the face of the Earth. They are also used as
base maps for recording and interpreting geologic and
environmental information that may be used in ana-
lyzing and solving environmental problems. The U.S.
Geological Survey, which has been actively engaged in
making topographic maps since 1882, has produced
maps covering practically all of the United States.
Appendix I contains useful conversion factors for
working with maps using data in both feet and miles
and meters and kilometers.
Interpretation of topographic maps is a great
deal easier if one has an understanding of the follow-
ing terminology:
In the Questions for Part A, the "Tour of a Topo"
section explains many of the additional types of infor-
mation that are on a topographic map.
Topography is the configuration of the surface of the
land.
19
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