"Sea surface height" is a time-varying quantity. A reference ellipsoid is a mathematical figure that approximates the geoid. The geoid is a surface of constant geopotential with which mean sea level would coincide if the ocean were at rest. The ellipsoid is an approximation because the geoid is an irregular shape. A number of reference ellipsoids are defined for use in the field of geodesy. The standard name for the height of the sea surface above the geoid is sea_surface_height_above_geoid. The standard name for the height of the sea surface above mean sea level is sea_surface_height_above_sea_level.