How often are index contour lines indicated on a topographic map?

Index contour lines on a topographic map are specifically designed to enhance the readability of elevation changes. These lines are typically marked with a bolder color or heavier line weight compared to regular contour lines, making them stand out. They are situated at regular intervals to provide a clear visual reference for elevation.

The correct answer indicates that every 5th contour line is designated as an index contour. This helps map users quickly identify the elevation values associated with the contour lines, facilitating easier navigation and understanding of the terrain features. The spacing of index contours allows for a balance between detail and clarity on the map, ensuring that those using it can effectively gauge the topography without overwhelming information from too many lines.