<P> A contour line (also isocline, isopleth, isarithm, or equipotential curve) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value . It is a plane section of the three - dimensional graph of the function f (x, y) parallel to the x, y plane . In cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level . A contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes . The contour interval of a contour map is the difference in elevation between successive contour lines . </P> <P> More generally, a contour line for a function of two variables is a curve connecting points where the function has the same particular value . </P>

The lines that join points of equal elevation above the mean sea level are known by what name