Each datum -- WGS-84, ED50, ... -- specifies one particular geoid, one model of the earth's surface. > Commonly an ellipsoidal model is part of a more encompas

Each datum -- WGS-84, ED50, ... -- specifies one particular geoid, one model of the earth's surface. > Commonly an ellipsoidal model is part of a more encompassing geodetic datum. https://en.m.wikipedia.org/wiki/Earth_ellipsoid https://en.m.wikipedia.org/wiki/Earth_ellipsoid A brief history of datum-geoid pairings can be found in the section Historical earth ellipsoids. For WGS-84 datum+geoid, an authoritative reference: http://www.unoosa.org/pdf/icg/2012/template/WGS_84.pdf http://www.unoosa.org/pdf/icg/2012/template/WGS_84.pdf