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

[Even Rouault]

> 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.
>

Cf https://github.com/OSGeo/PROJ/pull/3668 + 
https://github.com/OSGeo/PROJ-data/pull/97
>
> Another question is if it is a good idea to add a Geodetic TIFF Grid 
> for the transformation between the national levelling height system 
> NAP and LAT of the Netherlands to EPSG? And should we add that instead 
> of the ETRS89 – LAT transformation, or should we publish both in EPSG 
> to shorten the number of transformation steps in PROJ?
>
It is really up to you, the use cases you want to solve and also 
depending on if EPSG has a policy regarding this.  PROJ can 
automatically chain 2 GeogCRS->VertCRS transformation where the GeogCRS 
is the same (e.g doing EGM96 -> EGM2008, and I believe it can also chain 
a GeogCRS->VertCRS and VertCRS->VertCRS with a pivot common VertCRS. 
That said explicit concatenated transformations in the registry can help it.


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