Introduction to Topographic Maps

Topographic maps summarize the earth's topography, or the configuration of the landscape, in two dimensions. In the example shown below, brown contour lines (lines connecting points of equal elevation) indicate the shape of the land surface and the elevation above mean sea level in the Cheney, Washington area.

On a given topographic map, contour lines have a constant vertical spacing called the contour interval. On the map above, the contour interval is 10 feet, meaning that adjacent contour lines are separated vertically by 10 feet. The contour interval is reported at the bottom of a map, and may vary from one map to another. Also notice on the map above that every fifth contour line appears in bold and is labelled with its elevation; these are termed index contours.

The horizontal spacing of the contour lines on the map can suggest the steepness of the slope on the land surface. When contour lines lie close to one another the slope of the land surface is steep. When they are spaced out further apart the slope is gentle to nearly flat.

Contour lines are usually smooth curves, and never join one another. Note that when contour lines cross stream drainages, they become v-shaped and the apex points upstream. Contours are more rounded and u-shaped on ridges, with the curve pointing downhill. Depressions in the landscape, such as a sinkhole, are shown by closed contour lines with tiny hachure lines pointing in toward the center.

Incorporated areas on topographic maps are indicated by a reddish-pink overprint, while wooded areas are printed in green. Hydrologic features such as lakes and streams are printed in blue. Cultural features are printed in black. Various topographic map symbols are used to indicate specific features, for example, highways, railroads, powerlines, gravel pits, buildings, etc.

The standard issue topographic maps produced by the U.S. Geological Survey are referred to as 7.5 minute topographic quadrangles. These maps cover an area that is 7.5 minutes of latitude by 7.5 minutes of longitude, and are available for most all of the United States. The maps have a standard scale of 1:24,000 where one inch on the map is equal to 24,000 inches (2000 feet) on the ground. Other common topographic maps scales are 1:100,000 and 1:62,500, but show considerably less detail. You might consider why a 1:1 scale is seldom used when making a map.

Given this level of detail, topographic maps are very useful to geologists doing field work. Not only do they reflect the topography, but they also provide a detailed base on which field data can be mapped and plotted. They are also of interest to the general public, particularly hikers and outdoor sportsmen, because of their detail in representing the landscape. If you ever contemplate buying a home in the country, a topographic map would be useful in siting the house.

Lastly, the Public Land Survey System established in 1785 to subdivide land west of the Mississippi and Ohio Rivers is still in use today as a means of locating yourself on a map. Six-mile-square townships are bound by township and range lines (oriented north-south and east-west respectively) and are printed in red on a topographic map. Each mile-square section within a township is given a number, from 1 to 36, and each can be further subdivided into quarter sections. For example, in the diagram above, Salnave Park is located in the SW 1/4 of the SE 1/4 of Section 14.