Early mapmakers were faced with an interesting challenge. They had to figure out how to show elevation on a flat map. They decided to use an innovation called contour lines, which are lines drawn to indicate various elevations. These lines do not have beginning or end points but rather mark the landscape in irregular circles or elliptical patterns to show how an elevation follows the surface of the earth. These circles or ellipses become smaller as the elevation increases and are often quite narrow at the highest points.
Contour lines appear in groups controlled by contour intervals. If the terrain is only moderately elevated, the contour interval might be ten feet. Each line would represent a rise of ten feet in elevation. If the terrain is steeply elevated, cartographers might choose a larger contour interval, perhaps eighty or a hundred feet, to express a sharper rise. Then each line would represent a rise of eighty or a hundred feet in elevation. Most maps also have index contours marked with a number that label the elevation at that point. These lines are also shown in darker print.
The distance between contour lines on a map is not regular. If an area is fairly flat, for instance, and the contour interval is ten feet, the contour lines appear farther apart on the map because the ground slopes gently up, needing more distance to reach the next point of elevation. If an area is steeper, however, the contour lines on the map will be closer together, for the ground rises rapidly to the next ten-foot elevation mark. To review, more widely spaced contour lines show a shallower slope, while more closely spaced lines reveal a steeper slope.
Finally, some cartographers use colors to indicate various points of elevation. Either the lines themselves or the spaces between them may be colored to indicate increasing elevation. Contour colors usually move from cool colors like purple, blue, and green at lower elevations to warmer colors like yellow, orange, and red at higher elevations. This adds another layer of visual data to the map.