[QGIS-Developer] CRS problems with WGS84 and ETRS89

[Even Rouault]

On mercredi 25 septembre 2019 15:53:50 CEST Bo Victor Thomsen wrote:
> Thanks to Kristian and Even for some very thorough answers. That was
> somewhat heavy reading (for me at least) on such an ordinary Wednesday :-)
> 
> I'll guess that QGIS (or the underlying transformation library) is using
> some of the "earlier" realizations for both ETR89 and WGS84 datums since
> the  ETR89/UTM32N and WGS84/UTM32N coordinates of a given location is so
> close to each other (0.1 mm)

It doesn't use any particular realization. It just uses the generic datums for 
both, and thus get from the EPSG database, a null transformation between 
those.

$ projinfo -s EPSG:25832 -t EPSG:32632
Candidate operations found: 1
-------------------------------------
Operation n°1:

unknown id, Inverse of UTM zone 32N + ETRS89 to WGS 84 (1) + UTM zone 32N, 1 
m, Europe - ETRS89

PROJ string:
+proj=pipeline +step +inv +proj=utm +zone=32 +ellps=GRS80 +step +proj=utm 
+zone=32 +ellps=WGS84

The 0.1 mm you see comes from the fact that ETRS89/UTM32N uses the GRS80 
ellipsoid and WGS84/UTM32N the WGS84 ellipsoid, which differ by a tiny amount 
on their inverse flattenin. Given that we use a null transform between both 
datums, it is not significant at all.

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