Topographic Map Definition

A topographic map is a graphical representation of a portion of the Earth's surface including natural and cultural features. An important aspect of a topographic map is the use of contour lines to describe surface features.

Contour Lines

Contour lines connect points of similar elevation. If a contour line has an elevation of 100 meters, then all points on that line have an elevation of 100 meters.

Types of Contour Lines

There are typically three types of contour lines on topographic maps: index contour lines (dark brown lines), intermediate contour lines (lighter brown lines), and supplementary contour lines (dashed brown lines).

Contour Interval

The map legend of a topographic map gives the contour interval for that map. This is the change in elevation between intermediate contour lines.

Slope

Contour lines describe the slope of an area by their distance apart. Contour lines close together indicate a steep slope, while contour lines further apart indicate a gentler slope.

Describing Terrain Features with Contour Lines

The distance and direction of contour lines on a map indicate the terrain feature for the given area. Contour lines that form a series of concentric circles of increasing elevation indicate a hill. A V-shaped contour line, with the V pointing upward, indicates a valley.

References

• Idaho State University, Dept. of Geosciences: Introduction to Topographic Maps
• "Basic Land Navigation;" National Wildfire Coordination Group; 2007
• "FM3-25.26 Map Reading and Land Navigation;" U.S. Army; 2001

Resources

• United States Geological Survey: Topographic Maps
• United States Geological Survey: Topographic Map Symbols