[PROJ] How to add custom grid shift to proj.db?

[Even Rouault]

Peter,

> Suppose I wanted to create a compound CRS using a 2D projection + custom
> geoid file (let's call it foo.gtx), not included in proj-datumgrid. What
> table(s) in proj.db would I have to update so PROJ would know to open the
> right geoid file? I'm guessing `grid_transformation` is relevant here, but
> I'm not sure what else (maybe vertical_crs?).

Yes, you'd need to define a record in grid_transformation that relates a 
geographic 3D CRS (existing or to create) and a vertical CRS (existing or to 
create).

You can take inspiration for example from EPSG:9325 'NZGD2000 to NZVD2009 
height (2)' to define one.

Looking for "hBG18", I've found http://www.ngi.be/agn/FR/FR3.jsp . So the 
geographic crs would be ETRS89. I don't see in EPSG a vertical CRS that would 
correspond to DNG heights.
Perhaps our Belgium friends could help / contribute that to PROJ upstream and 
ask EPSG to add the required entries ?

Even

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