An ellipsoid typically defined by eight parameters which establish its dimensions, define its center with respect to the Earth's center of mass and specify its orientation in relation to the Earth's average spin axis and Greenwich reference meridian.
A mathematical model designed to best fit part or all of the geoid. It is defined by an ellipsoid and the relationship between the ellipsoid and a point on the topographic surface established as the origin of datum.
A set of parameters defining co-ordinate systems for all or parts of the earth. These datums have been refined and revised over time. NAD 27 is the North American datum for 1927, for example. ED50 is the European datum for 1950, and WGS84 is the World Geodetic System for 1984.
A means of representing the figure of the Earth, usually as an oblate ellipsoid of revolution, that approximates the surface of the Earth locally or globally, and is the reference for the system of geodetic coordinates.