[PROJ] RFC4: example of converted grid

[Even Rouault]

Hi,

You'll find updates in
https://github.com/rouault/PROJ/commit/038f488cbbba4ebb5ddc88e0cbc150b4e4f1f6c4

I've worked on a converter of NTv2 files to optimized GeoTIFF, including ones with
subgrids and applied it on	GDA94_GDA2020_conformal.gsb and ntv2_0.gsb (Canada NAD27->NAD83).
It also includes the samples containing offset accuracies. I've opted for a format
that will maximize efficiency for PROJ needs. That is: the various offsets (latitude offset,
longitude offset, latitude offset accuracy, longitude offset accuracy) are compressed in
separate blocks. Latitude and longitude offset blocks are interleaved so that when needing
correction terms for a given location they are consecutive. The accuracy samples are put
at the end of file.

Conversion scripts and sample datasets at https://github.com/rouault/sample_proj_gtiff_grids/

Compression can dramatically reduce file size in some cases.
For example GDA94_GDA2020_conformal.tif is 4.8 MB large (using DEFLATE compression),
to be compared to the 83 MB of the original file, and with the same sample values.

Even

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