I recently wrote some code to transform coordinates between WGS 84 and the new Swiss projected coordinate system (LV95 / EPSG:2056). While doing that, I tried to find an authoritative source for the definition of the Bessel ellipsoid, which is used for LV95.
I checked the EPSG registry, the website of swisstopo (national mapping agency of Switzerland), as well as some other sources (see references below).
It seems that everybody agrees on the first value:
Semi-major axis: a = 6377397.155 m
My understanding is that usually the semi-minor axis b or the inverse flattening 1 / f is usually specified in addition to a, or sometimes both, where one is derived from the other.
[EPSG-Nov2015] defines:
Inverse flattening: 1 / f = 299.1528128
But [swisstopo-Dec2016] defines:
Inverse flattening: 1 / f = 299.15281285
Semi-minor axis: b = 6356078.962822 m
Unfortunately [swisstopo-Dec2016] does not indicate which of the values is derived, but gives [Bolliger-1967] as a reference, which defines:
Semi-minor axis: b = 6356078.96282 m
([Bolliger-1967] does not specify which values are derived either)
Are these differences relevant? If so, is it a "bug" in one of the references? Or am I overthinking this and should I just pick one of the values at random?
References
- [EPSG-Nov2015] CH1903+ / LV95 (EPSG::2056)
- [swisstopo-Dec2016] Formulas and constants for the calculation of the Swiss conformal cylindrical projection and for the transformation between coordinate systems, swisstopo, December 2016 (see page 5)
- [Bolliger-1967] Die Projektionen der Schweizerischen Plan- und Kartenwerke, J. Bolliger, 1967 (see page 30)
