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

[Peter Gagliardi]

I am working with custom CRS transformations using cs2cs. For typical EPSG
codes, this works well, but I now want to support custom grid shift files
that are not listed in proj.db. I currently am unsure how to do this with
WKT strings.

I want to describe the following two types of transformations:


   1.

   Transform from horizontal CRS EPSG:XXXX to EPSG:4979 using yyyyy.gsb to
   transform heights
   2.

   Transform from EPSG:4979 to EPSG:XXXX using yyyyy.gsb to transform
   heights


Where yyyyy.gsb is a custom grid shift file. Is there a way to do this
using a COMPOUNDCRS like the following?

COMPOUNDCRS[“User-Defined Projection”,

     <WKT for horizontal CRS EPSG:XXXX>,
    VERTCRS[<whatever parameters are needed to specify yyyyy.gsb>]

]

Or do grid shifts require different syntax entirely? From the WKT2
specifications, it looks like PARAMETERFILE is relevant, but this looks
like it is specified separately from the CRS definition.

Thank you,

Peter Gagliardi
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