What does the line at 13600 represent? scale bar index contour elevation difference topographic symbol

Index contour

Explanation:

A line on a map at 13600 would likely represent an index contour. On a topographic map, an index contour is a contour which is usually thicker and bolder with its elevation value written on it. Using this index contour and the contour intervals, we can estimate the values of other contour lines on the map.

The use of an index contour makes the map less crowded with symbols thereby making the map readable.