Add Spilhaus projection #4401

jjimenezshaw · 2025-02-16T10:28:45Z

Implementation of the Spilhaus projection, following most of the recommendations done in #1851.

It is working for both the sphere and the ellipsoid. To support the ellipsoid it uses conformal latitudes.
An additional correction factor for the conformal latitude is included, that was not present in other implementations.

In ticket #1851 there was a discussion about the implementation in ESRI products, and the scale factor (apparently sqrt(2)) between both implementations. In case we want to do something else, please add a comment here. There is a CRS that uses it: ESRI:54099

Closes Spilhaus projection #1851
Tests added
Added clear title that can be used to generate release notes
Fully documented, including updating docs/source/*.rst for new API

jjimenezshaw · 2025-02-16T10:29:32Z

Before merging this PR I have to squash the commits (and probably address the comments).

rouault

Great work !

rouault · 2025-02-16T10:48:41Z

src/projections/spilhaus.cpp

+    PJ *adams_ws2;
+};
+
+double conformal_lat(double lat, double e) {


could we move that do a dedicated file in src/ ? (if it is not already there, but can't spot it)
I can see that mod_ster_setup() does this computation too and could be another user

Ok. Is there already a file with that kind of things, or better to a dedicated one?

Probably a dedicated one. We have some files phi2.cpp, mlfn.cpp, msfn.cpp, qsfn.cpp, tsfn.cpp with "similar" things but whose naming is apparently meant to be obscure enough that I never know what they do...
Or maybe have a "latitudes.cpp" file where we would group together code to transform between geographic latitude and all those "other latitudes ? But that may be work for another pull request

I created latitudes.cpp, and move the functions there.

src/projections/spilhaus.cpp

test/gie/spilhaus.gie

src/projections/spilhaus.cpp

rouault