In cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. A contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness of slopes. The contour interval of a contour map is the difference in elevation between successive contour lines.
Lieutenant Colonel William E. Persons, Infantry Major Oliver S. Wood, Infantry Major Joseph Plassmeyer, Cavalry Captain Paul V. Kellogg and , Infantry First Lieutenant John E.McCammon Military Science and Tactics (, MO: The University Co-operative Store, 1922)
Courtesy the private collection of Roy Winkelman