Affine transformation in proj < 6?

Hi all,

I'm new to proj, so sorry if this is a trivial question (I've read through docs for an hour without finding an answer, so it can't be too trivial).

I have a custom coordinate system, which is defined through an affine transformation from ETRS89. All coordinates I care about are in xyz Cartesian coordinates. In particular, to get pqr coordinates, I do

coords_pqr = (coords_etrs - pqrorigin_etrs) * (3x3 matrix)

Now I'm generating raster images in PQR, which I would like to store in GeoTiff, so if I understand correctly I need to write a proj string into the GeoTiff to describe the projection. More background: I'm writing the PQR-images in python, and want to export them so that I can load them into QGIS, with appropriate coordinates.

I see that an affine transformation is part of proj6, but I cannot expect users to have that installed. Is there a way to abuse another projection to achieve the same thing?

If there is another channel to ask this question, please let me know. Thanks in advance!

Tammo Jan Dijkema
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