» This article describes a concept from surveying and geodesy. For other meanings, see Datum (disambiguation).A datum (plural datums or data) is a reference from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the earth's surface against which position measurements are made, and (often) an associated model of the shape of the earth (ellipsoid) to define a geographic coordinate system. Horizontal datums are used for describing a point on the earth'… (More on Datum)