The system used to define the finite-difference approximation applied in the direction roughly perpendicular to the earth's surface in a numerical model. The most obvious example is the use of geometric height; however, there are numerical or physical advantages to many other vertical coordinate systems. For example, a “terrain following” vertical coordinate system is one with a lowest coordinate surface that follows the terrain. See sigma vertical coordinate, eta vertical coordinate, hybrid vertical coordinate, isentropic vertical coordinate, pressure vertical coordinates.