[PROJ] Krovak S-JTSK centimeter accuracy

[Even Rouault]

> 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
>
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