STUDENT INFORMATION
Rules for Reading and Creating Topographic Maps
Whether you are reading a contour map or creating one, keeping the
following rules in mind will make your work more accurate:
1. Mean sea level means an elevation of 0 feet (0 meters). A contour line
indicates one and only one elevation. EvelY point on a contour line
100 feet is 100 feet above sea level. (See the next figure .)
2. Contour lines never cross one another. (See the next figure.)
Not This
This
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What elevation?
3. All contour lines are closed figures; a line followed from point A will
eventually return to point A. Lines that run off the map will return to it.
(See the next figure.)
Map
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Rules for Topographic Maps
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STUDENT INFORMATION
4. Small closed loops indicate hilltops (See the next figure.) Hills or
mountains that are long and narrow are called ridges. These may
include a number of peaks. On a comour map, ridges look like long
oval contour lines.
Hilltop
5. The spacing of contour lines gives an indication of the slope. Generally
speaking, the closer the comour lines appear, the steeper the slope.
Gentle or shallow sloops have greater spacing. (See the next figure.)
Steep Slope
Gentle Slope
6. The shape of the contour lines indicates the shape of the feature as
viewed from overhead. (See the next figure.)
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Smooch surface
Irregular surface
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Rules for 1bpograpbic Maps
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STUDENT INFORMATION
7. The contour lines on a map are always spaced according to a standard
vertical distance. (See the next figure.)
50-foot interval
100
8. Where streams are shown on a topographic map, the contour lines
always form a "V'' or ''U" shape toward the head of the stream. The '"\...
shape points upstream toward higher elevation. (See the next figure.)
Head
500
Toward
400
mouth
300
200
9. Where the surface features a depression (such as a hole) hau:hnre
(HACH uhr) lines pointing in toward the center are added to
conwur
lines. Hatchure lines are short straight lines that repre;em a dep:'essicn.
(See the next figure.)
100
46
Rules for Topographic Maps
STUDENT INFORMATION
10. The only points on a contour map whose elevation is indicated precisely
are places whose exact elevation is indicated on the map-at a point
located on a contour line, at a benchmark, or at a spot elevation. A
benchmark is a point whose exact elevation is actually inscribed on a
brass or aluminum plate on the ground. Benchmarks are indicated on a
map by the letters BM, next to which the elevation (to the nearest foot) is
given. Spot elevations are the elevations of points of significance, such
as road forks, hilltops, and lake surfaces. These points are usually indicated on the map by a small cross. Numbers giving elevations checked by
surveyors are printed in black. Unchecked elevations are printed in
brown. Water elevations are shown in blue. (See the next figure.)
X
BM102
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Questions
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1. In your own words, defme a contour line.
2. Why is knowing the contour interval important in reading a contour or
topographic map?
3. How c'an you tell if a particular site is steep or shallow?
4. What values (measurements) are crucial in designing a topographic map?
5. What information do benchmarks and spot elevations provide for
the reader?
6. How are depressions or holes represented on a contour map?
7. Describe what a topographic map of your area would look like.
8. How can use of topographic maps aid in making good land-use decisions
in a community?
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Rules for 7bpographic Maps
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