[PROJ] Custom coordinate reference system

[Even Rouault]

Michel,

It would be fair for readers here if you mentionned that you already reported 
this to QGIS in
https://github.com/qgis/QGIS/issues/31106

and you got a detailed answer in
https://github.com/qgis/QGIS/issues/31106#issuecomment-558383488
explaining it was a QGIS limitation currently.

You could potentially add your own CRS and its transformation to WGS84 by 
hacking directly into the PROJ database. Relevant files in PROJ to edit would 
be data/sql/projected_crs.sql (and potentially geodetic_crs.sql and 
geodetic_datum.sql if there is no existing underlying Geographic CRS and datum 
in EPSG) and data/sql/helmert_transformation.sql. Or insert directly the 
entries in proj.db
You'd probably need to rerun QGIS crssync program afterwards

Hum actually as you specifically want the +exact keywords of proj=helmert, you 
should rather edit other_transformation.sql than helmert_transformation.sql, 
to add directly the pipeline transforming between the 2 underlying geographic 
CRS. A (really dummy) example of registering a PROJ pipeline string as a 
transformation is in
https://github.com/OSGeo/PROJ/blob/master/test/unit/test_factory.cpp#L2074

Even

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com