[PROJ] transformation for Greece

[Javier Jimenez Shaw]

Thanks!

On Mon, 3 Feb 2025 at 16:54, Even Rouault <even.rouault at spatialys.com>
wrote:

> Javier,
>
> you can probably hack around BOUNDCRS in a totally PROJ-specific way by
> using the "METHOD["PROJ-based operation method: {proj_pipeline_here}"
>
> For example with the following file, 'projinfo -s @/tmp/tmp.wkt -t "WGS
> 84"' will return the specified pipeline
>
> BOUNDCRS[
>     SOURCECRS[
>         GEOGCRS["unknown",
>             DATUM["Unknown based on GRS 1980 ellipsoid",
>                 ELLIPSOID["GRS 1980",6378137,298.257222101,
>                     LENGTHUNIT["metre",1]]],
>             PRIMEM["Greenwich",0,
>                 ANGLEUNIT["degree",0.0174532925199433],
>                 ID["EPSG",8901]],
>             CS[ellipsoidal,2],
>                 AXIS["longitude",east,
>                     ORDER[1],
>                     ANGLEUNIT["degree",0.0174532925199433,
>                         ID["EPSG",9122]]],
>                 AXIS["latitude",north,
>                     ORDER[2],
>                     ANGLEUNIT["degree",0.0174532925199433,
>                         ID["EPSG",9122]]]]],
>     TARGETCRS[
>         GEOGCRS["WGS 84",
>             DATUM["World Geodetic System 1984",
>                 ELLIPSOID["WGS 84",6378137,298.257223563,
>                     LENGTHUNIT["metre",1]]],
>             PRIMEM["Greenwich",0,
>                 ANGLEUNIT["degree",0.0174532925199433]],
>             CS[ellipsoidal,2],
>                 AXIS["latitude",north,
>                     ORDER[1],
>                     ANGLEUNIT["degree",0.0174532925199433]],
>                 AXIS["longitude",east,
>                     ORDER[2],
>                     ANGLEUNIT["degree",0.0174532925199433]],
>             ID["EPSG",4326]]],
>     ABRIDGEDTRANSFORMATION["Transformation from unknown to WGS84",
>         METHOD["PROJ-based operation method: +proj=pipeline +step
> +proj=axisswap +order=2,1 +step +proj=cart +ellps=GRS80 +step +proj=helmert
> +x=1 +step +inv +proj=cart +ellps=WGS84"]]]
>
> Otherwise having records in the DB will certainly be cleaner.
> data/sql/transformations_czechia_extra.sql can probably be used as an
> example of quite advanced transformations, including using a grid
> referenced in a projected CRS.
>
> Even
>
>
> Le 03/02/2025 à 16:26, Javier Jimenez Shaw via PROJ a écrit :
>
> 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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>
> -- http://www.spatialys.com
> My software is free, but my time generally not.
>
>
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