[PROJ] transformation for Greece

[Even Rouault]

Javier,

I don't think that can be done directly in PROJ. That would somehow 
require a "add" operator using coordinates pushed on the stack. Or maybe 
some super convoluted trickery abusing some of our existing operators, 
but nothing comes to mind.

No magic then: I would probably write custom code that would create a 
grid from ETRS89 to the final CRS (the geographic CRS of it), with 
roughly the resolution of the grid in TM07, applying "at hand" the 4 
steps to compute the shift in the lat/long space for all grid cells 
(using PROJ for some of the individual steps and Python to make the 
addition).

Even

Le 14/02/2025 à 17:50, Javier Jimenez Shaw a écrit :
> 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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>
>     -- 
>     http://www.spatialys.com
>     My software is free, but my time generally not.
>
-- 
http://www.spatialys.com
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