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<p>Hello all</p>
<p align="justify">PROJ does not seem to support the following
operation method at this time:</p>
<ul>
<li>EPSG:1026 — Mercator (Spherical) <br>
<a class="moz-txt-link-freetext" href="https://epsg.org/coord-operation-method_1026/Mercator-Spherical.html">https://epsg.org/coord-operation-method_1026/Mercator-Spherical.html</a><br>
</li>
</ul>
<p align="justify">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:</p>
<ol>
<li>If the ellipsoid is not a sphere, raise an error.</li>
<li>Otherwise treat as synonymous of EPSG:1024 — Popular
Visualisation Pseudo Mercator.<br>
</li>
</ol>
<p align="justify">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.</p>
<p align="justify">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.</p>
<p align="justify"> Martin</p>
<p align="justify"><br>
</p>
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