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's surface, in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths’. A datum is defined by the ellipsoid and the shift of the x,y,z origin.