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

[Lesparre, Jochem]

Nyall wrote:
> 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 :)

Yes, I noticed. It's good to have that option too. Although, I do not find it so easy to define a CRS like that, as WKT is not very compact and it is really hard for a human to get the parenthesis right. But, I willing to accept that if there is a way to refer to the exact Helmert transformation with WKT! Is this possible?

Kristian wrote:
> 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 :-)

I have no objection against editing the proj.db myself, however the CRS should be usable by other people too. My idea was to provide a simple instruction on how to add a custom CRS in QGIS, until I would get this CRS added to the EPSG registry. But I might need to get the exact Helmert method added to EPSG first.

Kristian wrote:
> Any chance I might be able to define my own PROJ transformation pipeline between a layer CRS and the project CRS then? That’d be handy from time to time and would probably also fix Jochems problem.

I'm not sure how this would work. I start to think that the best available solution is to create a hgridshift file. Unfortunately, that would give me the same horizontal coordinates at any height. Due to the large rotations of the CRS and a volcano in the area of interest, this would cause a horizontal error of 0.1 metre between points at sea level and the highest point. Unless, I create a really dense hgridshift file to let it give the right horizontal results at DTM height. But that would be bit complex.

Regards, Jochem



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