The height coordinate determined from GPS observations is related to the surface of a Reference Ellipsoid. The coordinates are derived initially in the 3-D Cartesian system (as XYZ values), and then for display/output purposes they are transformed to Latitude, Longitude and (Ellipsoidal) Height using well known formulae to an ellipsoid such as that associated with the WGS84 Datum (semi-major axis: 6378137m; inverse flattening: 298.257223563). The surface of the ellipsoid is the zero ellipsoidal height datum. In Relative Positioning, the height component of the receiver whose coordinates are being determined relative to the Base Station can also be related to an ellipsoid by transforming the baseline vector from the 3-D form (DXDYDZ) to a change in Latitude, change in Longitude, and change in Ellipsoidal Height.