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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