[PROJ] Defining a WKT CRS with a custom grid shift file

[Even Rouault]

> Ah, I wasn’t aware of BOUNDCRS until now. To check my understanding, a
> BoundCRS is one where you input coordinates in SOURCECRS (x, y, z). Then
> when PROJ reads this in, it uses the ABRIDGEDTRANSFORMATION to convert to
> TARGETCRS (x’, y’, z’) which becomes the effective source CRS. This can
> then be converted +to some other CRS as usual. Is this an accurate
> description?

I'd say that when wanting to transform the (geographic CRS underlying the) 
SOURECRS to TARGETCRS, then ABRIDGEDTRANSFORMATION is used.

> I also have a related follow-up question about some of the PROJ parameters.
> I now see that +geoidgrids=foo.gtx is for vertical shifts while
> +nadgrids=foo.gsb is for horizontal shifts. If this is the case, when would
> you use +proj=vgridshift +grids=foo.gtx?

+geoidgrids is a legacy syntax from the PROJ < 6 era when transformations 
where put in CRS definitions.

+proj=vgridshift +grids=foo.gtx is an operation part of a coordinate 
transformation pipeline to convert between CRS.

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