The geometrical separation between the geoid and the reference ellipsoid is called the geoidal undulation. VDs are used in astro-geodetic levelling, a geoid determination technique. Most global mean radii are based on the reference ellipsoid, which approximates the geoid. One other surface is involved in geodetic measurement&colon; the geoid. These deflection values were integrated into astrogeodetic geoid charts referred to these national datums. The permanent deviation between the geoid and mean sea level is called ocean surface topography. The difference between the ocean surface and the actual geoid gives ocean surface topography. On Earth, the zero elevation datum is based on sea level (the geoid). These geocentric ellipsoids are usually within of the geoid. Geostatistical approach has been defined as the most improved technique in prediction of geoid undulation. The geoid has no direct relationship with surface topography, however. Spherical harmonics are often used to approximate the shape of the geoid. The geoid height variation is under on Earth. The second step is to approximate the geoid by a mathematically simpler reference surface. The geopotential surface called the geoid is one definition of the shape of the Earth.