> If I use something defined with proj_create_crs_to_crs I get incorrect values for the meridian convergence
Which is as expected: proj_create_crs_to_crs returns a transformation BETWEEN two CRS. The geometrical factors are meaningfully defined for projections only. A transformation between two CRS is not in general a projection. You may get some numbers out of calling proj_factors with a transformation as an argument, but you should not expect any kind of meaningful result. Also, you may get numbers out of a PJ generated from a CRS (EPSG:nnnn), by ignoring the reference frame information implied, in which case an eventual projection remains. But if the CRS is, say, geographical or geocentric cartesian, the factors are meaningless. **It's apples and pears**. Don't expect meaningful proj_factors from anything other than a traditional projection specification.
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