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<p>Hi,</p>
<p>I've implemented this in <a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/pull/3741">https://github.com/OSGeo/PROJ/pull/3741</a></p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 25/05/2023 à 13:15, Martin
Desruisseaux a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:98ec7ccf-97ac-68a6-5479-414a70d2f6ee@geomatys.com">
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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"
moz-do-not-send="true">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>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
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</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
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