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

Unambiguous reference systems are a fun- damental requirement for accurate and reliable geode- tic products. The definition of the reference system, the realization by a reference frame, and the alloca- tion of the geodetic datum have to be strictly coher- ent. 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 ori- gin 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 coor- dinate transformations between selected reference sta- tions, 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 realiza- tion of the frame does not change the definition of the system.