Contour lines on topographic connect points of equal elevation on the earth crust.
They are used to delineate the elevation of the landforms in an area.
Index contours are heavier lines on the topographic maps. They have numbers for specific elevation. The elevation of the index contour is used in extrapolating those of other places on the map.
The difference in elevation between contour lines is called contour interval. Most maps usually have consistent contour interval.
