[PROJ] Geoid model in projected CRS

[Even Rouault]

Javier,


> Is there already a way to define a geoid model interpolated in 
> projected coordinates?
> (some "small" countries, with just one projection, define their geoid 
> models in that projected CRS. Recreating the grid in a geographic one 
> adds some error)
>
> I see that 
> https://proj.org/en/stable/operations/transformations/gridshift.html 
> was added in 9.2.0, but I only see these options:
> HORIZONTAL_OFFSET,
> GEOGRAPHIC_3D_OFFSET,
> ELLIPSOIDAL_HEIGHT_OFFSET,
> VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL, 
> VERTICAL_OFFSET_VERTICAL_TO_VERTICAL.
>
> I am not sure if any of those apply as 
> "VERTICAL_OFFSET_PROJECTED_TO_VERTICAL".

This is a bit untested area. VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL 
should be the one to use I believe.

> In case it is possible, how would be the insertion of the 
> transformation in proj.db? (something like 
> https://github.com/OSGeo/PROJ/blob/master/data/sql/grid_transformation_custom.sql#L71)

You likely need to use a PROJString with +proj=gridshift +grids=...

Even

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