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

[Nyall Dawson]

On Wed, 10 Feb 2021 at 04:56, Kristian Evers via PROJ
<proj at lists.osgeo.org> wrote:
>
> 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)

Actually since 3.10 we've also supported direct WKT(2) definitions of
custom CRS, and the GUI strongly pushes users towards defining custom
projections using WKT over PROJ now :)

Nyall

. 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> 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 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
>
>
>
> Disclaimer:
> De inhoud van dit bericht is uitsluitend bestemd voor geadresseerde.
> Gebruik van de inhoud van dit bericht door anderen zonder toestemming van het Kadaster
> is onrechtmatig. Mocht dit bericht ten onrechte bij u terecht komen, dan verzoeken wij u
> dit direct te melden aan de verzender en het bericht te vernietigen.
> Aan de inhoud van dit bericht kunnen geen rechten worden ontleend.
>
> Disclaimer:
> The content of this message is meant to be received by the addressee only.
> Use of the content of this message by anyone other than the addressee without the consent
> of the Kadaster is unlawful. If you have received this message, but are not the addressee,
> please contact the sender immediately and destroy the message.
> No rights can be derived from the content of this message.
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj
>
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj