[PROJ] How to do Helmert transformation with +exact option in QGIS?

[Lesparre, Jochem]

I want to use a 7-parameter transformation with sub-decimetre precision, but with quite large rotation angles (up to 480” = 8’), so I need to use the +exact option of +proj=helmert. Without +exact, I get discrepancies of several metres after transforming back and forth. This is a problem when defining a custom CRS in QGIS using a PROJ.4-style string, as it does +proj=helmert without +exact. Is there an equivalent of +exact for +towgs84?

An other option is to switch to a WKT string in QGIS to define the custom CRS. The autogenerated WKT string from the PROJ.4-style string refers to EPSG method 9606 which is the 3D Helmert transformation, not the exact variant. I had a look in the EPSG.org registry, but I could not find the rigorous 3D similarity transformation of +exact. Is there a way refer to the exact transformation with WKT?

Regards, Jochem



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