[PROJ] Switch to proj-datumgrid-geotiff for PROJ 7 ?

[Even Rouault]

Hi,

This is one of the last major topic linked to RFC4: should we switch to 
GeoTIFF grids as the prime format for the grids we deliver alongside the 
software? While there is no strong technical requirement to do so, it could be 
the right moment to do it, or we might prefer postponing that to PROJ 8.

Currently RFC4 implementation continues to refer to .gtx / .gsb files in the 
grid_alternatives database table. When network access if allowed and needed, 
the code patches the extension to access a .tif file on the CDN.

It would probably be cleaner to avoid this patching and having 
grid_alternatives directly point to .tif files. This would also enable us to 
put https://github.com/osgeo/proj-datumgrid in a pure maintainance state and 
just make https://github.com/osgeo/proj-datumgrid-geotiff the new home for 
grids (currently the later has to be resynchronized with the former)

Currently:
proj-datumgrid: total size: 703 MB as 5 .zip and 1.5 GB uncompressed
proj-datumgrid-geotiff: total size: 486 MB

One potential issue with switching to GeoTIFF files is for people having 
pipeline strings using the current grid names in them. But similarly to what 
is currently done for remote access, for local access, if we fail to find a 
.gtx/.gsb file, we could just retry with the .tif extension.

If we switch to proj-datumgrid-geotiff, would we still split the content in 
several archives (and if so, according to which logic...) ? Having a single 
archive would be for sure the simplest solution. Some feedback from packagers 
would be welcome.

A bit linked to the above, if we switch to proj-datumgrid-geotiff, in the 
grid_alternatives table, instead of having each grid entry pointing to a 
package (like "proj-datumgrid-europe", which itself points to a URL of a .zip 
archive), we could just point to the exact file on the CDN rather than the 
archive.

If we adopt proj-datumgrid-geotiff, I'd also willing to add a proj-datumgrid-
download utility that would download, in the user-writable directory (or 
system directory), files from the CDN based on criteria such as bounding box, 
producer name, country of the producer, etc...

Even

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