[PROJ] Dealing with epoch and transformations

[Even Rouault]

 > So, I agree with you that using the reference epoch, like PROJ does, 
is the most reasonable thing to do for the moment.

Just to clarify that if you use a time-dependent Helmert transformation 
without specifying an epoch, PROJ will only apply the non-time-dependent 
terms. So the "reference epoch" in that situation is the one of the 
transformation, not the frame reference epoch of the datum

Cf:

$ echo 1 0 0 | bin/cct +proj=helmert +x=1 +dx=.1 +t_epoch=1234 2.0000 
0.0000 0.0000 inf

$ echo 1 0 0 1235 | bin/cct +proj=helmert +x=1 +dx=.1 +t_epoch=1234
2.1000 0.0000 0.0000 1235.0000

PROJ 9.5 has now a new API function, 
proj_coordoperation_requires_per_coordinate_input_time(), to indicate 
you if a given coordinate operation expects time-dependent terms: 
https://github.com/OSGeo/PROJ/pull/4221 .

There's also an explicit error code for the transformations where the 
coordinate epoch is really required, so currently +proj=defmodel and 
+proj=deformation +t_epoch= . For time-dependent Helmert, this is 
considered optional as mentioned above (mostly for backward compatibility)

Even

-- 
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