[PROJ] geoid model from a geographic 2D

[Javier Jimenez Shaw]

Thanks Even.

Until I move this with the SLA, the workaround of a new entry in
geodetic_crs works fine (and the corresponding in usage)

On Tue, 26 Mar 2024 at 15:43, Even Rouault <even.rouault at spatialys.com>
wrote:

> Javier,
> > 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.
> Geoid models make only sense between a Geographic 3D CRS  and a Vertical
> CRS or Geog 2D CRS + Vertical CRS, so the check here is relevant. You
> need an ellipsoidal height at the source end. You could try to remove
> the check, but I don't think that's a good idea. I'm pretty sure there's
> logic in different places of the code that expect a Geographic 3D CRS in
> that situation.
> >
> > 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).
>
> Would be the preferred solution, and also a record for their geoid model
>
> Your workaround pending that is to add an entry in the geodetic_crs
> table (under the PROJ authority for now) to create a Geographic 3D CRS
> companion to EPSG:4757.
>
> Even
>
> --
> http://www.spatialys.com
> My software is free, but my time generally not.
>
>
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