What is the primary purpose of contour lines on a map?

The primary purpose of contour lines on a map is to show surface elevation. Contour lines are used to represent the three-dimensional shape of the terrain on a two-dimensional surface. Each line connects points of equal elevation above a specific reference point, usually sea level. By examining the spacing and shape of these lines, one can determine how steep or flat the terrain is, as well as identify features such as hills, valleys, and plateaus.

When contour lines are close together, they indicate a steep slope, while lines that are far apart represent a gentle slope. This visual representation of elevation is crucial for various applications, such as outdoor navigation, land use planning, and understanding geographical features.