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

[Even Rouault]

Le 17/03/2023 à 15:44, Lesparre, Jochem a écrit :
>
> I wrote:
>
> > To prevent users to make different choices in similar use cases, the documentation should indicate what to do 
> for Lowest Astronomical Tide (LAT) or other Chart Datum grids.
>
> ER wrote:
>
> > Cf https://github.com/OSGeo/PROJ/pull/3668 + 
> https://github.com/OSGeo/PROJ-data/pull/97
>
> Thanks! This means that the Geodetic TIFF Grid for Chart Datum of 
> Norway (and possibly also the one of Denmark that Kristian Evers is 
> working on) wrongly use type VERTICAL_OFFSET_VERTICAL_TO_VERTICAL 
> instead of VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL. But as that works 
> too, it’s not a big deal.
>
> I have two questions:
>
>   * Are you sure about the tag “hydroid_height”? A Google search
>     (https://www.google.com/search?q=%22hydroid+height%22) gives only
>     1 relevant hit. I have no opinion on this, as it is not my
>     expertise. I’m just helping the people at our hydrographic service.
>
This is borrowed from the 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). 
I assume they are right, as they are much smarter people than me. They 
have a slightly different naming convention "hydroidHeight" which I 
modified to match our convention.
>
>  *
>
>
>   * Why limit the description to only predefined options? I can
>     imagine that one could want to use a reference surface that is not
>     a geoid nor a hydroid, for example an ellipsoidal DTM grid to
>     transform to/from Above Ground Level heights. So it might be
>     easier to allow any description to prevent future problems.
>
Well, no, we must have a predefined and limited list. Otherwise PROJ 
grid interpolation code, when inspecting a GeoTIFF dataset, will not 
know in which channel to find the information.


-- 
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