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

[Nyall Dawson]

On Tue, 26 Mar 2019 at 15:25, Kristian Evers <kristianevers at gmail.com> wrote:
>
>
>
> > On 26 Mar 2019, at 00:47, Even Rouault <even.rouault at spatialys.com> wrote:
> >
> >> (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…
> >
>
> I absolutely agree with Even here. This behaviour is odd and not what I
> expected as user of QGIS. Now I wonder if I have introduced errors
> somewhere in the past because I wasn’t aware of this (probably not, I
> mostly work with GRS80 and WGS84).

I honestly don't have any plans to change this behavior. I cannot see
a way to remove this choice from users, as QGIS projects don't have a
single CRS, but they DO need a single ellipsoid. The best we can do is
make sure that a good default selection is given to users, but we
cannot totally remove this choice.

Looking into this a bit, there is issues with the current matching of
initial CRS to ellipsoid. I'm hopeful the move to proj 6 will fix this
(the current code seems unable to correctly determine the ellipsoid
for some projections).

Nyall




>
> /Kristian
>
>
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