What is the purpose of index contours on a topographic map?

Index contours serve a critical function in topographic maps by providing a clearer and more informative means of interpreting the elevation and terrain of the mapped area. These contours are typically drawn at regular intervals, often every fifth contour line, and are usually labeled with their corresponding elevation values. This visual distinction allows map readers to quickly ascertain elevation changes and better understand the landscape's slope and relief.

By highlighting significant elevation points, index contours simplify the process of reading a topographic map, allowing users to easily discern features such as hills, valleys, and plateaus in the terrain. This effectively enhances the interpretation of contour maps, which can be complex due to the numerous lines representing varying elevations. Consequently, the function of index contours aligns perfectly with the need for clearer interpretation of the depicted data related to the physical geography of the area.