Glossary for Geospatial Science

geoid:  The figure that represents the irregular spheroidal shape of the earth is called the geoid.  The geoid is an equipotential surface (which means the direction of gravity is perpendicular to all points on the surface) at mean sea level.  Because of variations in the distribution of the mass and density of the earth’s constituents, the geoid generally rises over the continents and is depressed in oceanic areas ;  the geoid also conforms to various lumps and depressions that depart from average smoothness on the earth’s surface.  For mapping purposes, the attributes found on the geoid must be transferred to a more regular geometric surface, an ellipsoid.