<div dir="ltr"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">What do you mean by local: a grid that deal with projected coordinates ? Is it really the case here ?<br></blockquote><div>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.</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">OK, so "standard grids" here. Are they the same you referred too previously ?<br></blockquote><div>They are different from the published files mentioned previously that are in projected coordinates. These are grids that PROJ knows how to handle.</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">I would strongly suggest that you, or more appropriately the Czech geodesic authority register those grids with IOGP in the EPSG dataset.<br></blockquote><div>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.</div><div><br></div><div>Thanks for your insight, Even.</div><div>Sincerely, Erixen</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Mar 16, 2022 at 2:07 PM Even Rouault <<a href="mailto:even.rouault@spatialys.com">even.rouault@spatialys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<div>First question: </div>
<div>Does Proj support this local coordinate grid shift?</div>
</div>
</blockquote>
<p>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.<br>
</p>
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<div><br>
</div>
<div>Second question:</div>
<div>The Czech Technical University in Prague has a page about
using Proj to transform to/from S-JTSK <a href="http://freegis.fsv.cvut.cz/gwiki/S-JTSK_/_Grid" target="_blank">here</a> (<a href="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" target="_blank">English</a>). It suggests using grid
shift files from some CZ researchers with the old `nadgrids`
and `geoidgrids` paradigm. </div>
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</blockquote>
OK, so "standard grids" here. Are they the same you referred too
previously ?<br>
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<div>would Proj accept a PR for adding these grid shift files to
get the better accuracy?</div>
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</blockquote>
<p>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 <a href="https://github.com/OSGeo/PROJ-data/" target="_blank">https://github.com/OSGeo/PROJ-data/</a>, you'll have
also to make sure that they are licensed under an open data
license such as the ones listed in
<a href="https://github.com/OSGeo/PROJ-data/#about-the-data-package" target="_blank">https://github.com/OSGeo/PROJ-data/#about-the-data-package</a>. 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.</p>
<p>Even<br>
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<div><br>
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<div>Thanks for reading, and thanks for any advice you can
offer.</div>
<div>Sincerely,</div>
<div>Erixen Cruz</div>
<div>3D Software Developer, <a href="https://cesium.com/" target="_blank">Cesium</a></div>
<div><a href="mailto:erixen@cesium.com" target="_blank">erixen@cesium.com</a></div>
</div>
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