Definition

The FIGURE OF THE EARTH considered as a mean sea level surface extended continuously through the CONTI­NENTS. The actual geoid is an EQUIPOTENTIAL SURFACE to which, at every point, the PLUMB LINE (direction in which GRAVITY acts) is perpendicular. It is the geoid which is obtained from observed DEFLECTIONS OF THE VERTICAL and is the surface of reference for ASTRONOMI­CAL OBSERVATIONS and for GEODETIC LEVEL­LING. See REFERENCE SPHEROID.