Map Interpretation
TO P O G R A P H I C M A P S
The Map
A topographic map is a widely used tool for graphically depicting variations in elevation within an area. A contour line connects
points of equal elevation above some reference datum, usually
mean sea level. A vast storehouse of information about the relief
and the terrain can be interpreted from these maps by understanding the spacing and configuration of contours. For example, elevations of mountains and valleys, steepness of slopes, and the direction of stream flow can be determined by studying a topographic
map. In addition to contour lines, many standard symbols are used
on topographic maps to represent mapped features, data, and information (a guide to these symbols is in Appendix B).
The elevation difference represented by adjacent contour
lines depends on the map scale and the relief in the mapped
area, and is called the contour interval. Contour intervals on
topographic maps are typically in elevation measurements
divisible by ten. In mountainous areas wider intervals are
needed to keep the contours from crowding and visually merging together. A flatter locality may require a smaller contour interval to display subtle relief features. It is good practice to note
both the map scale and the contour interval when first examining a topographic map.
Keep in mind several important rules when interpreting
contours:
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Closely spaced contours indicate a steep slope, and widely
spaced contours indicate a gentle slope.
Evenly spaced contours indicate a uniform slope.
Closed contour lines represent a hill or a depression.
Contour lines never cross but may converge along a vertical cliff.
A contour line will bend upstream when it crosses a valley.
Interpreting the Map
© Bruce Perry, Department of Geological Sciences, CSU Long Beach
1. What is the contour interval on this map?
2. The map scale is 1:24,000. One inch on the map represents
how many feet on the Earth’s surface?
3. What is the highest elevation on the map? Where is it located?
4. What is the lowest elevation on the map? Where is it located?
5. Note the mountain ridge between Boat and Emerald Canyons
(C-4). Is it steeper on its east side or its west side? What led
you to your conclusion?
6. In what direction does the stream in Boat Canyon flow? What
led you to your conclusion?
Aerial photograph of the coast at Laguna Beach, California.
7. The aerial photograph below depicts a portion of the topographic map on the opposite page. What area of the air photo
does the map depict? How well do the contours represent the
physical features seen on the air photo?
8. Identify some cultural features on the map. Describe the
symbols used to depict these features. The map shown is
older than the aerial photograph. Can you identify some
cultural features on the aerial photograph not depicted on
the contour map?
Opposite:
Laguna Beach, California
Scale 1:24,000
Contour interval = 20 feet
U.S. Geological Survey
Opposite: © Bruce Perry, Department of Geological Sciences, CSU Long Beach
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