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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

swisstopo Bessel definition

Bolliger Bessel definition

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I found the authoritative source with the help of @swiss_geoportal and @swisstopo on Twitter.

It's an ordinance called Verordnung des Bundesamtes für Landestopografie über Geoinformation (GeoIV-swisstopo) (not available in English).

The following value for the flattening of the Bessel ellipsoid is provided for both reference systems used in Switzerland (CH1903 and CH1903+):

f = 1/299.15281285

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