[PROJ] transformation for Greece

[Javier Jimenez Shaw]

Hi Even.

The example in data/sql/transformations_czechia_extra.sql was very useful.
First I am trying to reproduce the transformation with a pipeline, and
later I will make the proper operations and concatenated operations in the
database.

However this Greek transformation is more convoluted than I expected
(initially I misunderstood some steps).

a) First they have a Helmert from ETRS89 (from their realization, but I do
not care) to GGRS87. So far so good (params are not the ones from EPSG, but
no problem)
b) Then they apply the transverse Mercator to "GGRS87 / Greek Grid". All
fine.
c) In parallel they project ETRS89 coordinates into a different transverse
Mercator, called TM07 (not in EPSG; they provide the params). The grid with
the horizontal corrections are in TM07
d) the final projected coordinates in "GGRS87 / Greek Grid" are the values
from b) plus the corrections from the grid in c)

See that the easting-northing coordinates to get the correction from the
grid are not the same as the coordinates from b). To make it clear, TM07
has a false-northing of -2000 km

How can it be done in a pipeline? (later to be converted into something in
proj.db)
I do not know how to keep the corrections somewhere to be added later to
the projected coordinates.

Thanks
Javier


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}"
>
> 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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>
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>
>
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