[PROJ] [EXTERNAL] Re: Proj 6 API questions

[Even Rouault]

> (Disclaimer: this functionality has been around much longer than I've
> been involved with QGIS, so I may be missing some of the background
> here)
> 
> QGIS exposes the choice of ellipsoid on a per-project level. This
> project ellipsoid is independent of the map CRS[1][2], so a project in
> WGS84 could have an ellipsoid of GRS80, and a project in Web Mercator
> could use the WGS84 ellipsoid. The project ellipsoid is used ONLY when
> calculating geodetic distances and areas of features. (It's also
> possible to set no ellipsoid, which means distances/area calculation
> is always purely Cartesian). It's also possible for users to enter a
> custom ellipsoid for a project, if they have some compelling reason to
> do so!
> 
> So we need a way to store the project ellipsoid within the project. In
> current QGIS versions that's done by the ellipsoid name, from QGIS'
> custom ellipsoid db. When we move to proj6 api I'm replacing this with
> the unique auth:code combination.
> 
> Does that help clarify?

Yes, thanks. OK, I see it now in Properties/General/Measurement.
Defining an ellipsoid that is not the same as the ellipsoid of the per-project 
CRS makes no sense to me. And I see this is what happens if opening for 
example a raster in UTM NAD27 in an empty project: the ellipsoid is set (or 
likely let to its default value of) WGS84 ! Aouch ! If it was me, I would just 
kill this definition of a custom ellipsoid...

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