Horizontal datums determine the geometric relations between a coordinate system grid and a particular ellipsoid approximating the Earth`s surface. Vertical datums determine elevation reference surfaces, like mean sea level. A. Horizontal datums. Relation of coordinate system to particular ellipsoid, datum transformation options, Molodensky and Helmert transformation, other high accuracy transformations, ED50 and WGS84, historical development of horizontal datums, ETRS89. B. Vertical datums. Historical development of vertical datums, difference between vertical datum and geoid, relations between ellipsoidal (geodetic) heiht, geoidal height and orthometric elevation.