A contour line is that line that on a map joins all points that have equal conditions, usually altitude above sea level or depth. Contour lines are usually printed on maps in sienna "Sienna (color)") for terrain and in blue for glaciers and deep sea. The impression of the relief is usually accentuated by giving shading that simulates the shadows that the relief would produce with lighting from the North or Northwest. On wall maps, the surfaces between two agreed contour lines are printed with certain conventional inks (hypsometric inks). For example: dark green for the depressions "Depression (geography)") located below sea level, increasingly lighter greens for medium altitudes, and increasingly intense siennas for high altitudes, reserving red or violet for the highest peaks on earth.[1]​.

In Geodesy, each of the contour lines materializes a horizontal section of represented relief. The equidistance, difference in altitude between two successive curves, is constant and its value depends on the scale of the map "Scale (cartography)") and the importance of the relief.

In Oceanography the isobat is a curve that is used for the cartographic representation of points of equal depth in the ocean and in the sea, as well as in large lakes.