There are contour lines in a topographic map. A contour line is an imaginary line on the surface of the Earth connecting points of equal elevation.

The vertical spacing (difference in elevation) between contour lines is known as the contour interval. Typical contour intervals are 10, 20, 50, and 100 feet. Every fifth contour is printed darker. These lines are called index contours. If the contour interval on a map is 20 feet, then the 100', 200', 300' (etc.) contours will be index contours.