The fundamental surface in Geodesy. It is defined as the equipotential surface of the gravity field that most closely approximates the Mean Sea Level. (The MSL deviates from the Geoid surface by 1-2 meters due to the Sea Surface Topography caused by wind-driven or geostrophic currents.) The Geoid is the Vertical Datum surface both from a mathematical viewpoint (i.e., the sum of the Orthometric Height and the Geoid Height equals the Ellipsoidal Height of a point), as well as in practice by making the land height system synonymous with "height above MSL". Models of the Geoid Height have been determined from the combined processing of satellite-derived potential models, surface gravity observations and the ocean gravity anomalies derived from Satellite Altimetry. Their accuracy may range from a few meters in the open ocean areas, down to the few decimeter level in land areas where there is a good coverage of surface gravity.