[PROJ] Krovak S-JTSK centimeter accuracy

[Erixen Cruz]

>
> What do you mean by local: a grid that deal with projected coordinates ?
> Is it really the case here ?
>
Yes, it is in projected coordinates with corrections in the X and Y
directions (easing, northing). I was thinking of reprojecting to geodetic
coordinates to be able to use `hgridshift`, but there is nuance/complexity
like you mentioned.

OK, so "standard grids" here. Are they the same you referred too previously
> ?
>
They are different from the published files mentioned previously that are
in projected coordinates. These are grids that PROJ knows how to handle.

I would strongly suggest that you, or more appropriately the Czech geodesic
> authority register those grids with IOGP in the EPSG dataset.
>
That makes sense. These "standard grids" come from researchers from Czech
universities, and the most accurate one has an MIT-like (open) license. We
can just use it by adding it to the PROJ_LIB directory with the rest of the
other files that come with proj-data.

Thanks for your insight, Even.
Sincerely, Erixen


On Wed, Mar 16, 2022 at 2:07 PM Even Rouault <even.rouault at spatialys.com>
wrote:

>
> First question:
> Does Proj support this local coordinate grid shift?
>
> What do you mean by local: a grid that deal with projected coordinates ?
> Is it really the case here ? (sorry didn't have time to read thoroughly
> your links). We've had instances where the data producer originally
> provided grids referenced to projected coordinates, like the OSGB15 geoid
> model, and the contributor to PROJ-data reprojected them to geographic
> coordinates to make them more usable by PROJ. Of course this only worked
> for a geoid model. If the content of the grid itself is shifts in eastings,
> northings, then a more complex transformation has to be done. Otherwise
> PROJ would need a few changes to deal with grids that would operate on
> projected coordinates.
>
>
> Second question:
> The Czech Technical University in Prague has a page about using Proj to
> transform to/from S-JTSK here
> <http://freegis.fsv.cvut.cz/gwiki/S-JTSK_/_Grid> (English
> <https://freegis-fsv-cvut-cz.translate.goog/gwiki/S-JTSK_/_Grid?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp>).
> It suggests using grid shift files from some CZ researchers with the old
> `nadgrids` and `geoidgrids` paradigm.
>
> OK, so "standard grids" here. Are they the same you referred too
> previously ?
>
> would Proj accept a PR for adding these grid shift files to get the better
> accuracy?
>
> I would strongly suggest that you, or more appropriately the Czech
> geodesic authority register those grids with IOGP in the EPSG dataset.
> That's the main source of geodesic data for PROJ. Otherwise we need to add
> custom entries, and this adds a maintenance burden to the project. If you'd
> want those grid(s) to be included in https://github.com/OSGeo/PROJ-data/,
> you'll have also to make sure that they are licensed under an open data
> license such as the ones listed in
> https://github.com/OSGeo/PROJ-data/#about-the-data-package. It can take
> some time of discussion with the data producer so 1) they are aware about
> the existence of licenses 2) they are willing to distribute under an open
> data license.
>
> Even
>
>
> Thanks for reading, and thanks for any advice you can offer.
> Sincerely,
> Erixen Cruz
> 3D Software Developer, Cesium <https://cesium.com/>
> erixen at cesium.com
>
> _______________________________________________
> 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.
>
>
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