Agriculture Reference
In-Depth Information
map ScaLeS and contour intervaLS
USGS maps come in several scales. Scale is the relationship between dis-
tance on the map and distance on the ground. A map at 1:24,000
2
(1 inch
= 2000 feet) shows considerable detail useful for planning and design
purposes. The maps follow a standard graphic representation format, a
format established early in the twentieth century. Older maps provide
useful historical information, snapshots of land development over time
when earlier maps are compared with more recent ones.
Map contours graphically represent the shape and elevation of the
terrain. The contours represent a system of elevations, with the elevation
difference from one contour to the next being equal throughout the map.
The elevation difference is called the contour interval. Contour intervals
vary, depending on the terrain and the scale of the map. For instance, the
contour interval for a 1:24,000 map, where there are considerable elevation
differences, such as in a mountainous region, the contour interval might
be 20 feet. For example, a sequence of contours in steep terrain might be
140´, 160´, 180´, and so forth. The contour interval is printed in the margin
of each U.S. Geological Survey map. In the example that follows, the ter-
rain is gently rolling, so the contour interval is 10 feet. Where the topog-
raphy of an area is relatively flat, such as in the Central Plains or coastal
wetlands, the contour interval might be 5 or 10 feet, so that a sequence of
contours might be 120´, 130´, 140´, and so forth.
Topographic contours on a USGS map are shown as brown lines of
different widths or opacity. To aid in more easily determining elevations,
index contours (such at 100-foot intervals) are darker and wider, with
lighter, thinner lines for intermediate 20-foot contour intervals. Eleva-
tion values are printed in several places along these lines. The narrower
intermediate and supplementary contours found between the index con-
tours help show more details of the land surface shape. Contours that are
very close together represent steep slopes. Widely spaced contours or an
absence of contours means that the ground slope is relatively level. The
elevation difference between adjacent contour lines, called the contour
2 To convert a USGS quad map to an engineer's scale, consider a USGS map of 1:24,000 scale, where
one inch equals 24,000 inches. Divide 12 into 24,000 to get feet. The result is 1 inch equals 2000 feet.
