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

[Kristian Evers]

> 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).

/Kristian


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