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<p>Hi,</p>
<p>Cf <a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/issues/2739">https://github.com/OSGeo/PROJ/issues/2739</a>,
<a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/pull/3010">https://github.com/OSGeo/PROJ/pull/3010</a> and
<a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ-data/pull/76">https://github.com/OSGeo/PROJ-data/pull/76</a> for a similar case for
Norway</p>
<p>Using VERTICAL_OFFSET_VERTICAL_TO_VERTICAL / vertical_offset
isn't super appropriate as this is intented for VerticalCRS to
VerticalCRS but I see that's what was used in the Norway case.
PROJ isn't super pedantic about that and just checks that
geoid_undulation or vertical_offset are found in the band
description. Looking at the beloved GGXF draft
(<a class="moz-txt-link-freetext" href="https://raw.githubusercontent.com/opengeospatial/CRS-Gridded-Geodetic-data-eXchange-Format/master/specification/GGXF%20v1-0%20OGC-22-051r2_2023-01-09.pdf">https://raw.githubusercontent.com/opengeospatial/CRS-Gridded-Geodetic-data-eXchange-Format/master/specification/GGXF%20v1-0%20OGC-22-051r2_2023-01-09.pdf</a>),
they have distinguished geoid and hydroid cases with a
"hydroidHeight" parameter, but ultimately the maths are the same.
We could potential extend PROJ to accept hydroidHeight as a valid
band description name.</p>
<br>
<blockquote type="cite"
cite="mid:AM0P192MB0482E5CC15C4621FDEA861B4EFBC9@AM0P192MB0482.EURP192.PROD.OUTLOOK.COM">
<div class="WordSection1">
<p class="MsoNormal"><span lang="EN-GB">Another, thing I was
wondering is how to deal with the difference between height
and depth. Should the Geodetic TIFF Grid specify ETRS89 +
LAT NL depth (EPSG:9289) as target CRS, so EPSG would only
have to make transformation code and a tif equivalent of the
transformation method “</span><span
style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#303030;background:white"
lang="EN-GB">Geog3D to Geog2D+Depth (txt)” (</span><span
lang="EN-GB"><a
href="https://epsg.org/coord-operation-method_1115/Geog3D-to-Geog2D-Depth-txt.html"
moz-do-not-send="true" class="moz-txt-link-freetext">https://epsg.org/coord-operation-method_1115/Geog3D-to-Geog2D-Depth-txt.html</a>)?
</span></p>
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</blockquote>
<p>That should be sufficient. That's what is used for the Norway
case (for the 'Geographic3D to Depth (Gravsoft)' method)</p>
<p>Even</p>
<p><br>
</p>
<pre class="moz-signature" cols="72"><a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
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