[PROJ] transformation for Greece

[Even Rouault]

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.
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj

-- 
http://www.spatialys.com
My software is free, but my time generally not.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20250203/c7658806/attachment-0001.htm>