[PROJ] geoid model from a geographic 2D

[Javier Jimenez Shaw]

Hi

I am trying to add a geoid model for Singapore to my proj.db.
The problem is that the "source crs", SVY21 (EPSG:4757), is only defined as
Geographic 2D in EPSG
https://spatialreference.org/ref/?search=SVY21

If I add something like this (copied from other cases in
grid_transformation_custom.sql)

INSERT INTO "grid_transformation" VALUES(
    'PROJ','EPSG_4757_TO_EPSG_6916','SVY21 to SHD height',
    NULL,
    'EPSG','9665','Geographic3D to GravityRelatedHeight (gtx)',
    'EPSG','4757', -- source CRS (SVY21)
    'EPSG','6916', -- target CRS (SHD height)
    NULL,
    'EPSG','8666','Geoid (height correction) model file','SGeoid09.tif',
    NULL,NULL,NULL,NULL,NULL,NULL,NULL,0);

I get an error:
One grid_transformation with Geographic3D to GravityRelatedHeight or Geog3D
to Geog2D+XXX has not its source_crs in geodetic_crs table with type =
'geographic 3D'

I am using the method 9665 ... just because I am used to it. I could use a
different one. But I see that all available methods are expecting a
geographic 3D as source... that will trigger the same error.

Is there any way to workaround this condition?

Asking SLA to add a 3D system to EPSG is one way... but I would like to
have a solution earlier (or in case they just don't do it).

Thanks,
Javier.
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