[PROJ] Add trivial support for EPSG:1026 operation method?

[Even Rouault]

Hi,

I've implemented this in https://github.com/OSGeo/PROJ/pull/3741

Even

Le 25/05/2023 à 13:15, Martin Desruisseaux a écrit :
>
> Hello all
>
> PROJ does not seem to support the following operation method at this time:
>
>   * EPSG:1026 — Mercator (Spherical)
>     https://epsg.org/coord-operation-method_1026/Mercator-Spherical.html
>
> Would it be possible to add it? (I can contribute a patch if I get 
> some guidance about which file to edit). The implementation can be 
> very trivial:
>
>  1. If the ellipsoid is not a sphere, raise an error.
>  2. Otherwise treat as synonymous of EPSG:1024 — Popular Visualisation
>     Pseudo Mercator.
>
> Step 1 could be replaced by the radius of the conformal sphere as 
> suggested by EPSG, but the above trivial implementation would be 
> sufficient for now for the purpose described below.
>
> The rational for adding EPSG:1026 support would be for the definition 
> of extra-terrestrial CRS. For compatibility with existing software, 
> some members of the OGC Planetary working group want to use the 
> Pseudo-Mercator projection. However many extra-terrestrial CRS are 
> defined on a sphere rather than an ellipsoid. In that particular case, 
> Pseudo-Mercator (EPSG:1024) is mathematically equivalent to Spherical 
> Mercator (EPSG:1026). The CRS could use former, but I would encourage 
> the planetary group to use the latter because it carries a semantic 
> difference. The EPSG guidance notes warns the reader about the 
> problems of Pseudo-Mercator (non-conformal, etc.). However those 
> issues do not apply when the datum uses a sphere. The EPSG guidance 
> notes do not mention that fact maybe because Pseudo-Mercator has been 
> introduced for use with ellipsoids and is useless on spheres (because 
> the existing classical Mercator projection was already doing the exact 
> same thing). The use of Pseudo-Mercator on a sphere may give to 
> non-experts the false impression that Pseudo-Mercator problems still 
> apply, while actually it got the properties of a classical Mercator 
> projection, in particular the fact that it become a conformal 
> projection. Using "Mercator (Spherical)" operation method instead 
> makes that fact clearer. Given that the implementation can be trivial 
> — identical to Pseudo-Mercator with only a check for making sure that 
> the datum uses a sphere — it seems to me that it would be a reasonable 
> addition to what can be expected from software (not only PROJ), and 
> that the gain in semantic value is worthy.
>
>     Martin
>
>
>
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