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

[Kristian Evers]

Jochem,

There is no +towgs84 equivalent of +exact. One of the many reasons to not use the old style syntax. Unfortunately that doesn’t solve your problem with QGIS. Unfortunately the QGIS custom CRS is still based on the PROJ.4 way of thinking (as far as I know, it’s been a while since I last tried it). If it is super important to you, you may be able to hack the proj.db that QGIS opens and add your own custom CRS and transformations. Not sure I would recommend that though :-)

Unfortunately I can’t answer your WKT question. Perhaps you can investigate how the WKT of other CRS that require the +exact flag is implemented?

Best,
Kristian

On 7 Feb 2021, at 21:36, Lesparre, Jochem via PROJ <proj at lists.osgeo.org<mailto:proj at lists.osgeo.org>> wrote:

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<http://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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