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The older Swedish three-dimensional reference systems SWEREF 93 and RR 92 are described here.
SWEREF 93
SWEREF 93 is the predecessor of the reference system SWEREF 99. The reason why SWEREF 93 has been replaced is that the computation was made before EUREF decided on how to do the computation. SWEREF 99 was computed according to the guidelines and is the current realisation of ETRS89 in Sweden.
The difference between SWEREF 93 and SWEREF 99 is 5–6 centimetres.
Facts about SWEREF 93
- The system is related to the ellipsoid GRS 1980, geocentrically located.
- The pole is CTP (Conventional Terrestrial Pole, mean pole around 1900).
- Coordinates are first calculated in the global system ITRF91, epoch 1993.6 with the VLBI station Wettzell – that is connected to Onsala – as a fixed point (the provisional system "DOSE 93A").
- "DOSE 93A" is then fitted to the European system EUREF 89 with a three-dimensional Helmert transformation with fixed scale. 11 points, of which 5 Swedish, were used in the fitting.
- The system apparently refers to the tide-free Earth (with elastic deformation factor).
- The plate tectonic epoch is 1989.
SWEREF 99 – the successor
With the introduction of SWEREF 99 (2001‑05‑26), SWEREF 93 can be considered obsolete. More information about SWEREF 99.
RR 92
The national reference system 1992, RR 92, is an older "spurious" three-dimensional system based on Bessel's ellipsoid.
The reason why RR 92 is called spurious, is that it is a pure joining of the horizontal system RT 90, the geoidal height system RN 92 and the height system RH 70.
RR 92 was created to be able to handle three-dimensional coordinates at an early stage. As SWEREF 93 – and later SWEREF 99 – were developed, RR 92 shall no longer be used.
Placement of the ellipsoid
The origin, i.e. centre, of RR 92 was placed less than one kilometer from the earth's centre of gravity. The placement was chosen to get a good national fit to the geoid.
References
- Ekman M.: Earth ellipsoids, geoids, coordinate systems, altitude systems and gravity systems in Sweden through the ages, LMV report 1998:4 (pdf, in Swedish, new window).
- Reit B-G.: SWEREF 93 – a new Swedish reference system, LMV report 1994:28 (pdf, in Swedish, new window).