[PROJ] Combining custom transformations with EPSG

[Even Rouault]

> > - ETRF2000_TO_NKG_ETRF00 using a fixed central epoch would need to be
> > another kind of operation. It would be a bit like the time-specific
> > Helmert transformation that is used in a few EPSG transformations (but we
> > currently don't map to a PROJ transformation).
> 
> I am not sure I follow, are you thinking about a WKT2-mapping of
> +proj=deformation? Or perhaps an EPSG-mapping? At some point I will get
> round to submitting these transformations to the EPSG registry.

Yes I was talking about EPSG mapping ( WKT itself doesn't define any name of 
operation method )

> Another solution is to not use NKG_ETRF00
> as a hub at all and simply make one big concatenated operation from ITRF2000
> to each national realization of ETRS89. 

Yes, you would to the 1-hop handled case.

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