[PROJ] transformation for Greece

Wed Feb 26 02:51:58 PST 2025

Hi

Thanks Even for the confirmation.
As a workaround I was thinking that in this case I can apply the grid
before the Helmert transformation. It is not what they say in the specs,
but the max error is about 0.1 mm

I tested it with pyproj. Considering that the checkpoints have an accuracy
of 1 mm, it is already matching it ;)

The pipeline would be something like this:

pipeline = """+proj=pipeline
    +step +inv +proj=cart +ellps=GRS80
    +step +proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000
+y_0=-2000000 +ellps=GRS80 +units=m
    +step +proj=gridshift +grids=gr_hepos_tm07_grid.tif
    +step +inv +proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000
+y_0=-2000000 +ellps=GRS80 +units=m
    +step +proj=cart +ellps=GRS80
    +step +proj=helmert +x=203.437 +y=-73.461 +z=-243.594
        +rx=-0.170 +ry=-0.060 +rz=-0.151
        +s=-0.294 +convention=coordinate_frame
    +step +inv +proj=cart +ellps=GRS80
    +step +proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=0
+ellps=GRS80 +units=m
"""

In case anyone is interested, the info about the workflow is in
https://www.hepos.gr/%cf%83%cf%85%cf%87%ce%bd%ce%ad%cf%82-%ce%b5%cf%81%cf%89%cf%84%ce%ae%cf%83%ce%b5%ce%b9%cf%82/
more in particular in
https://cdn.ktimatologio.hast.gr/HEPOS_transformations_200219_484787931f.zip
It includes the grid file in ascii.

Unfortunately the system GGRS87 is only defined in 2D in EPSG:
https://spatialreference.org/ref/?search=ggrs87

Cheers.
Javier

On Sat, 15 Feb 2025 at 16:35, Even Rouault <even.rouault at spatialys.com>
wrote:

> 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.
>>
>> _______________________________________________
>> PROJ mailing listPROJ at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/proj
>>
>> -- http://www.spatialys.com
>> My software is free, but my time generally not.
>>
>> -- http://www.spatialys.com
> My software is free, but my time generally not.
>
>
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