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

Contour lines on a map serve the crucial function of illustrating the relief and elevation of the terrain. These lines connect points of equal elevation, allowing users to visualize the shape and steepness of the landscape. By interpreting the spacing and configuration of contour lines, one can discern features such as hills, valleys, and plateaus. Close contour lines suggest steep terrain, while lines that are spaced farther apart indicate gentler slopes. This representation is invaluable for various activities including hiking, land use planning, and geological studies, as it provides critical information about the physical features of an area.

The other options focus on different aspects of map information: distances, water bodies, and residential areas, which do not pertain to the specific purpose of contour lines.