in geodesy, a surface where the gravitational potential is the same at all points. The direction of the normal to an equipotential surface coincides with the direction of the force of gravity, that is, with the plumb line. An example of an equipotential surface is the surface of a liquid in equilibrium. The equipotential surface of the earth’s gravity field coincides with the mean water level in the oceans; this surface is called the geoid and is taken as the mathematical surface of the earth, or the “sea level,” from which the heights of points of the earth’s surface are measured. The form of the geoid is highly complex and depends on the internal structure of the earth.

a surface all of whose points have the same potential. For example, the surface of a conductor in electrostatics is an equipotential surface. In a force field the lines of force are normal, or perpendicular, to an equipotential surface.