contour or contour line, line on a topographic map connecting points of equal elevation above or below mean sea level. It is thus a kind of isopleth, or line of equal quantity. Contour lines are drawn on maps with a uniform interval of vertical distance separating them (usually 10, 20, 50, or 100 ft on American maps) and thus outline the landform configuration, or relief. They may be visualized as representing shorelines if sea level were raised in small increments. Thus the tops of hills, which would appear as separate islands, are shown as a series of closed circular contours; valleys, which would appear as elongate bays, are shown as contour lines converging toward a point at the head of the valley. Since on steep slopes there is little horizontal distance between points greatly different in height, contour lines indicating such terrain are close together; contour lines of gentle slopes are more widely separated. Maps employing contour lines are called contour, or relief, maps although they are popularly called topographic maps (see topography) in the United States. Certain conventions are employed on these maps to assist the user. Contours indicating land elevations are printed in brown with every fifth contour drawn thicker and labeled with its elevation; those indicating depths of bodies of water are printed in blue. Hachure lines, pointing downslope, are attached to contour lines in order to emphasize a depression with a steep gradient. In the past, contour maps were made from ground surveys. Today they are constructed from stereographic aerial photographs and orbiting satellites, which use radar to measure elevations for land or ocean relief maps. In an analogous way, contour lines are also commonly used to map properties other than elevation; meteorologists, for example, employ contour lines, or isobars, to delineate areas of equal barometric pressures.