Contour Lines. Contour lines are the most common
method of showing relief and elevation on a standard
topographic map. A contour line represents an imaginary
line on the ground, above or below sea level. All points on
the contour line are at the same elevation. The elevation
represented by contour lines is the vertical distance above
or below sea level. The three types of contour lines (Figure
10-1) used on a standard topographic map are as follows: