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Contour, Contour Lines, and Contour Intervals Defined
Contours give the overall arrangement of the landscape an asymmetrical, curving appearance. Topographic maps demonstrate these contours using contour lines, which link land surface points with identical elevations. Contour spacing depicts the vertical distance, or the change in elevation, between contour lines.
Contour Lines
The contour lines on topographic maps include a few features for easier map reading. Contour lines generally are brown in color, indicating they're earth features, and setting them off from other features such as water, trees, buildings or roads. Every fifth contour line, called an index contour, appears thicker and includes the elevation --- in feet or meters --- listed at intervals along the line. The four intermediate contour lines in between are thin and frequently don't include elevations, especially if the contour interval is narrow. Contour lines forming concentric circles show hilltops or mountaintops.
Contour Intervals
Contour intervals represent stacked, horizontal slices of terrain between contour lines. The map legend lists the vertical distance of these contour intervals as well as the map scale. The map scale establishes the contour interval since the height of any land feature is directly related to the horizontal distance of nearby map features. If a contour interval is listed as 40 feet, the difference between contour lines is 40 feet in elevation, and the elevation difference between thicker, index contour lines becomes 200 feet. Contour intervals are also determined by the land feature's slope and intersection with other land features such as rivers and lakes.
Contour Interval Spacing
Generally, smaller map scales generate larger contour intervals. On any topographical map, narrow contour intervals indicate steep slopes or cliffs, whereas widely spaced contour lines denotes gentle slopes. Contour intervals intersect streams and valleys in a V-shape with the apex pointing upstream.