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

[Lesparre, Jochem]

Hi PROJ mailing list,

I am making a Geodetic TIFF Grid for the transformation between ETRS89 and Lowest Astronomical Tide of the Netherlands (https://epsg.org/crs_9289/ETRS89-LAT-NL-depth.html).

So this Geodetic TIFF Grid is of the type "VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL". The description "geoid_undulation" seems unfitting, but if I choose something else like "lowest_astronomical_tide" PROJ doesn't accept it:
proj_create: Error 1029 (File not found or invalid): pipeline: Pipeline: Bad step definition: proj=vgridshift (File not found or invalid)
cct: Bad transformation arguments - (File not found or invalid)
    'cct -h' for help
Should I use the description "geoid_undulation" nevertheless, should I use something else for type instead of "VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL", or should I make a feature request to support a description name for "hydroid" models?


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)? Or should the Geodetic TIFF Grid specify ETRS89 + LAT NL height (for which no EPSG code is available yet) as target CRS? I case of the latter, EPSG would need to make transformation code and an equivalent of the transformation method "Geog3D to Geog2D+GravityRelatedHeight (txt)" (https://epsg.org/coord-operation-method_1098/Geog3D-to-Geog2D-GravityRelatedHeight-txt.html), but also have to add an additional CRS code for LAT NL height, a Compound code ETRS89 + LAT NL height, and a transformation code LAT NL height to LAT NL depth based on "Height Depth Reversal" (https://epsg.org/coord-operation-method_1068/Height-Depth-Reversal.html).

Best regards, Jochem



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