Undulation of the geoid

Undulation of the geoid is the mathematical process of determining the height in metres above the geoid (relative to the mean sea level) from the height provided by the GPS system which uses the (WGS84) ellipsoid as reference. In maps and common use the height over the mean sea level is used to indicate the height of elevations while the ellipsoidal height results from the restrictions that apply for the GPS system.