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

[Even Rouault]

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?

A clean solution would indeed to have EPSG register a method for an exact 
Helmert transformation and that could then be used as an abridged 
transformation of a BoundCRS

In the mean time, I've implemented something that enables to embed an 
arbitrary PROJ string pipeline in a BoundCRS:
https://github.com/OSGeo/PROJ/pull/2521
Of course that will not be interoperable outside of PROJ.

Even

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