[gdal-dev] GDAL 3, PROJ 6.2 exportToProj4() drops Datum silently

[Nyall Dawson]

On Wed, 30 Oct 2019 at 06:16, Even Rouault <even.rouault at spatialys.com> wrote:
>
> > I realise this too, but to permit legacy results to be replicated,
>
> Some legacy behaviour was just plain wrong/inaccurate. Trying to replicate it
> in PROJ would be heartbreaking, an extra maintaince burden, and a confusion
> for PROJ users because depending on the path they take, they would get
> different results.
>
> Not-so-random example with GDAL 2.4/PROJ 5 with GDA94 to GDA2020
>
> You get a ~1.5 metre shift to the north east as expected.

This thread has been bouncing around in my mind for the last week.
Forgive me if I'm misinterpreting the situation, but wouldn't you
explicitly want legacy results NOT to be reproducible in cases where
proj 6's more accurate transformation capabilities have affected these
results?

If a particular study isn't affected by relatively small shifts such
as the 1.5m shift in the GDA2020 situation, then the results should
remain reproducible even if this shift is corrected. But if a
particular study IS affected by distances of this magnitude then I'd
argue that it would be vital that the older proj behavior can't be
reproduced exactly, and accordingly the previous study/results are
invalidated as a direct result..!

Nyall


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