How to "read" a contour map

What most distinguishes USGS topographic maps is the use of brown contour lines to show the shape and elevation of earth's surface. Contours are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface, usually mean sea level. Contour maps afford greater precision than other pictorial or shaded relief maps. Only a contour map permits you to determine, to an accuracy within the the contour interval, the elevation of any point on the map.

When you learn to read contour maps, you become able to visualize accurately in three dimensions hills, valleys, cliffs, craters, and more. If your mind's eye needs practice, simply get a large scale topographic map of a familiar piece of ground that is not flat, go there, and spend some time comparing actual shapes of the land with their depictions on the map.

oblique view and contour mapHere is a picture and a contour map of the same landforms. In this example, the contour interval is 20 feet. On USGS maps the contour interval is always stated in the center of the bottom margin. Different contour intervals are used on different maps depending on the type of terrain and the scale of the map. On most topographic maps every fifth contour is a bolder line labeled with its elevation.

If sea level rose 20 feet, the new shoreline would be the first brown contour line next to the original shoreline. If it rose 100 feet, the new shoreline would be the the fifth, or the first bold contour line. Note that where the land slopes steeply, contours are close together, and in flat areas the contours are far apart. Wherever two or more contours touch is a vertical cliff. Contour lines can never cross each other. If you get confused about the direction of slope, look for the nearest body of water- it will be at the bottom of a slope. Where contours cross ravines and stream valleys you see a "V" shape, with the V always pointing upstream.

contour map with kettleholeContours that describe a depression or crater are distinguished by short dashes called tick marks, which point downhill. The map segment at right has a contour interval of 10 feet. In the northern (upper) half is a depression that could conceivably fill with water to a depth of over 30 feet. South of it is a small hill. Without the tick marks, the depression would look like a hill.

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