Reference Systems, Reference Frames, and the Geodetic Datum - Basic Considerations

Unambiguous reference systems are a fundamental requirement for accurate and reliable geodetic products. The definition of the reference system, the realization by a reference frame, and the allocation of the geodetic datum have to be strictly coherent. In traditional geodetic reference systems used in triangulation and trilateration networks, the datum was given through independent (astronomic) observations in selected fundamental stations, which fixed the origin and the orientation of the coordinate system. The datum of modern geocentric reference systems must also be determined by independent methods, namely by gravity field parameters and physical models. If it is derived from the reference frame itself, i.e., by coordinate transformations between selected reference stations, the definition of the system will be changed: It does no longer refer to the geo-centre but to the centre of the reference network. Therefore it is indispensable to strictly pay attention that the datum is not affected by the measurements of the frame, and that the realization of the frame does not change the definition of the system.