How to read a topographic map

topographic map

Most of the maps we are familiar with, such as road maps or political boundary wall maps, are planimetric maps, that is they show the world as if it were on one plane with the observer directly overhead. Understanding these maps is a simple matter of reading (the legend tells what the map symbols represent) and arithmetic (the scale indicates distances, as in one inch represents 10 miles).

A topographic map represents the land as it appears in three dimensions as if the observer was looking at eye level across the landscape. The idea for topographic maps dates back to the 1600s. In France, finance minister Jen Baptiste Colbert initiated a complete topographic mapping of his entire country by hiring noted astronomer Jean Dominique Cassini to identify elevations of mountains, depths of valleys, and location of manmade objects. It would take more than a century and four generations of Cassinis to complete the massive project but when it was completed, France became the first country to have its entire landscape recreated on topographic maps.

The United States Geological Survey (USGS) produced America’s first topographic map in 1879, the year the agency was created. The USGS is still producing topographic maps today although aerial photography and satellite transmissions long ago replaced the dedicated cartographers that painstakingly plotted each map by hand.

To create its topographic representation of the United States, the USGS divided the contiguous 48 states and Hawaii into 54,000 quadrangles (map sheets). A separate set of topographic maps represent Alaska. On these maps, one inch equals 24,000 inches or 2000 feet. Each map sheet is physically about 29 inches high and 22 inches wide. To read a topographic map, it is necessary to know the principles behind the symbols on the map.

The first key is to understand the color codes on a topographic map. All manmade structures are represented in black. Water is always blue and areas of vegetation green. Major highways are lines of red and boundaries of public land are shaded in red. Purple objects are unchecked objects that have been added to an original survey after aerial surveillance. The brown lines are contour, or elevation lines.

These contour lines are the lifeblood of topographic maps and there are several tips in understanding their meaning:

*Concentric contour lines that form complete, closed paths, whether those paths constitute circles or some irregular shape, represent mountains and hills.

*Contour lines in valleys form a ‘V’, the point of which heads to higher ground.

*The closer the contours are spaced, the steeper the terrain.

*A path straight up or down a slope will always be represented on a map by a straight line perpendicular to the contour lines depicting that slope.

*Thin contour lines are intermediate contours and every fifth contour is a thick index contour which has an elevation height written on it. You can be certain whether a formation is a ridge or a valley by checking these contour numbers to see if they are descending or ascending.

*When the tips of two contours point at each other, there is a pass in between.

*Where contour lines merge there is a cliff, with a distinct change in elevation but no horizontal distance.

Topographic maps can be ordered from the United States Geological Survey by writing to the Easter Distribution Branch at 1200 S. Eads Street, Arlington, Virginia 22202 for maps east of the Mississippi River and the western Distribution Branch at Box 25286, Federal Center, Denver, Colorado 80225 for maps west of the Mississippi.

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