[PROJ] Lowest Astronimical Tide reference surface in Geodetic TIFF Grid

[Even Rouault]

Hi,

Cf https://github.com/OSGeo/PROJ/issues/2739, 
https://github.com/OSGeo/PROJ/pull/3010 and 
https://github.com/OSGeo/PROJ-data/pull/76 for a similar case for Norway

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 
(https://raw.githubusercontent.com/opengeospatial/CRS-Gridded-Geodetic-data-eXchange-Format/master/specification/GGXF%20v1-0%20OGC-22-051r2_2023-01-09.pdf), 
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.


> 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 “Geog3D to Geog2D+Depth (txt)” 
> (https://epsg.org/coord-operation-method_1115/Geog3D-to-Geog2D-Depth-txt.html)? 
>
>
That should be sufficient. That's what is used for the Norway case (for 
the 'Geographic3D to Depth (Gravsoft)' method)

Even


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