Here's why they are important:
* Elevation Reference: Index contours are labeled with their elevation, providing a clear reference point for determining the elevation of other features on the map.
* Visual Distinction: The heavier weight makes them stand out from the other contour lines, making it easier to quickly identify and read the map.
* Elevation Interval: Index contours typically represent a larger elevation change than the standard contour lines, often a multiple of the standard interval. This helps to highlight major changes in elevation.
So, the next time you're looking at a topographic map, remember that those thick, labeled lines are your key to understanding the landscape's elevation!
