Contour lines on a map indicate elevation or depth. Numeric notations on contour lines refer to the level of elevation, or height above sea level.
Some contour lines on maps are left unlabeled. The height above sea level indicated by contour lines that are not labeled can be calculated by using a map's contour interval, which is the difference between neighboring contour lines. Elevation of points between contour lines can be estimated by observing the distance to the closest two contours.
Contour lines are one of several methods used in cartography to demonstrate the general characteristics of terrain. They are used at various scales for large engineering drawings and smaller maps.