[PROJ] Proj transformation for arbitrary (quaternion/Euler) spatial rotation

[DAVEN P QUINN]

Thanks to everyone in this conversation for their suggestions.

It seems to me that there are two approaches that have been suggested that may work: using the `lat_p` and `lon_p` options to `ob_tran`, and using a pipeline that converts to geocentric cartesian coordinates, does a Helmert transform (consisting entirely of rotation), and converts back to angular space.

I have also tried the “move the pole location” signature of ob_tran (by computing the new location of the pole based on my desired spatial rotation), and it does seem to do a reasonable spatial shift, but there is a rotational component that is not properly accounted for in that method. I have tried to handle that last degree of freedom by a combination of `lon0` and `alpha`, but I think the alpha in this case does a rotation in Cartesian space.

The Helmert option seems potentially viable (though slightly less preferred because pipelines are only possible in the most recent PostGIS version) but I was under the impression that was used mostly for small angular shifts of data already in projected (i.e., 2 or 2.5d) space, not rotating a full geocentric manifold through a large angle. I will try it out though.

Another option seems like it might be two stages of ob_tran pole movement, first moving the north pole of the projection to the pole of rotation, changing the longitude around that pole, and then moving the pole back to its computed final location. It seems like getting the math right in this case would be challenging.

I will continue exploring in these directions, and read the code Evan provided for more context. Any additional thoughts on this synthesis would be appreciated.

Regards,

Daven
On Nov 9, 2022, 5:42 PM +0100, Even Rouault <even.rouault at spatialys.com>, wrote:


How does Proj actually do the G.O.T? I don't mean what are the
parameters, I mean what is the actual code that does the arithmetic?

==> https://github.com/OSGeo/PROJ/blob/master/src/projections/ob_tran.cpp


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