[PROJ] [EXTERNAL] Re: package 'geosphere' -- transforming spherical-based geographic coordinates to WGS84

[Even Rouault]

David,

That could be the matter of days of discussions. I'll try to make it short :-)

- the +proj= syntax you mention for EPSG definitions has been deprecated since PROJ 6, 
since it was a rather imperfect way of capturing datum expressions. With GDAL 3 & PROJ >= 
6, we encourage strongly to use directly EPSG:XXXX syntax instead of deprecated PROJ.4 
strings

- ... but for the code you mention, the datum behind  EPSG:3408 is datum EPSG:6053 "Not 
specified (based on International 1924 Authalic Sphere)" and the one for EPSG:3411 is EPSG:
6054 "Not specified (based on Hughes 1980 ellipsoid)". So even with PROJ 6, there's no hope 
to get any significant datum transformation

- EPSG:3408 has been recently deprecated by EPSG in favor of EPSG:6931, and EPSG:3411 in 
favor of EPSG:3413. Both use the WGS84 datum, and are thus not equivalent to their non-
deprecated code.

- With PROJ.4 syntax, there's no toXYZ qualifier, only towgs84. WGS84 in PROJ.4 era was 
used as the universal pivot when transforming between 2 datums (sometimes it was abused 
a bit to transform directly between a CRS A and B when A or B was sufficiently close to 
GRS80/WGS84 for the approximation to be valid). Starting with PROJ 6, one can do direct 
transformations, but with your rather unspecified datums, I've no idea what would be the 
right way of transforming them...

Even

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