What is an index contour line on a topographic map?

An index contour line on a topographic map is a specific contour line that is marked and labeled with an elevation value, typically representing a significant interval between elevations. These lines help to enhance the readability of the map, allowing users to quickly identify the elevation at various points.

In practice, index contour lines are usually drawn at regular intervals, such as every 5th contour line. By providing this information, they serve not only as visual aids to the terrain's elevation but also as reference points for distance and slope when navigating or analyzing the map's features. Being able to interpret these lines is crucial for understanding the topography represented, making it easier to determine the lay of the land and for applications such as hiking, land development, and geological studies.