On a topographic map, a contour line that is thicker than the rest and usually labeled with the elevation that it represents. Depending on the contour interval, every fourth or fifth contour line may be an index contour.
On a topographic map, a contour line that is thicker than the rest and usually labeled with the elevation that it represents. Depending on the contour interval, every fourth or fifth contour line may be an index contour.