[PROJ] Alternate use model on EPSG v10+ datum ensembles

[Even Rouault]

> The GEOID99 example helped me understand that in EPSG gridded data
> transformation extents don't have to encompass the (larger) extents of the
> source & target CRS.  So the "chart datum" realizations may be a singular
> CRS/datum construct, and N transformations may be attached to that src/tgt
> per tiled extents of gridded data therein.

That would definitely work. One point of attention though is that PROJ only 
considers the bounding box of the transformation, and not the potentially more 
complex polygonal shape. So if the grids have overlaps, they'd better have 
compatible values otherwise the one that will be picked up by PROJ will be 
"random" (or more precisely, it will be deterministic but hardly predictible 
for mortals) if they have the same declared accuracy.

> Can an EPSG transformation parameter point to a
> GeoTIFF TIFF Grids model file containing several grids, and PROJ would
> handle lookup/interpolation of the appropriate grid on-the-fly?

Yes that would work. (There's no declared method yet in EPSG for a TIFF based 
grid AFAICS, but that should be just a matter of allocating a method number 
for it.)

> I'm not
> aware of any multi-grid storage formats referenced by EPSG outside of NTV2,
> but those nodes hold horizontal corrections for lat & lon, rather than
> vertical shift values.

The PROJ vgridshift operation should work fine with GeoTIFF grids made of 
several base grids. The NTV2 mechanism of several base grids and nested 
subgrids has been ported to the GeoTIFF format and for all currently handled 
grid-based transformation.

To quickly create a composed grid from several grids (GDAL >= 3.0):
gdal_translate first_grid supergrid.tif
gdal_translate second_grid supergrid.tif -co APPEND_SUBDATASET=YES

To make it cloud-friendly, see
https://github.com/OSGeo/PROJ-data/tree/master/grid_tools#optimize-an-existing-geotiff-file

And then use it with
cct +proj=vgridshift +grids=supergrid.tif


Even

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