[PROJ] transformation for Greece

[Javier Jimenez Shaw]

Hi

I am trying to make this transformation (that is not in EPGS). I do not
know how far I can go with PROJ.

(Apparently this transformation is needed to go from the coordinates used
by HEPOS.gr, the Greek positioning system, to the official -I think- CRS in
Greece, GGRS87.
The best is that the transformation would be in EPSG. Do you know anybody
in Greece with the proper contacts? I am happy to talk with them about it,
and help if needed)

Until it is in EPSG, I would like to use PROJ to do it.

The accurate transformation from ETRS89  to GGRS87 is something like:
- apply a Helmert transformation
- Project into a transverse Mercator
- Apply a correction grid. Yes, in the projected system.
- (optional) go back from the projected to the geographic crs.

I thought about several options:

- First option: create a WKT2 with a BOUNDCRS that defines the
transformation. Can it be done in a single WKT2?
- Second option: add the transformation to proj.db. Is it doable?
- Third option: Create "my own grid" file (in the geographic system)
combining all the steps. Of course, it will loose some accuracy due to the
new grid interpolation.

Any clue?
Thanks.
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