Index Contour
Index contour explained in an easy to understand way:
Imagine you're trying to navigate a crowded concert and you spot a friend wearing a bright, easily identifiable hat. Just like that standout hat helps you locate your friend in the sea of people, an index contour on a map acts as a standout line that shows you a specific elevation in a sea of other contour lines. Just as the hat helps you find your friend quickly, the index contour, being a heavier and labeled line, helps you quickly identify key elevation levels on a topographic map, making it easier to understand the terrain.
Practice Version
Index Contour: On a map, the heavy contour line labeled with the elevation. Index contour. An index contour is a thicker line on a map that shows elevation and helps you easily identify height levels in terrain.