[PROJ] Geoid model in projected CRS

[Javier Jimenez Shaw]

Hi

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".
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
)

Thanks.
Javier.
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