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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