[gdal-dev] Upgrading to GDAL 3.4.2 from 1.10.0 shows different outputs from gdaltransform and corresponding C++ API

[Greg Troxel]

Even Rouault via gdal-dev <gdal-dev at lists.osgeo.org> writes:

> 3) You would get the """"most precise result"""" (with lots of quotes,
> because this is highly subjective, and assumes that NAD83(HARN) is
> equivalent to WGS 84, according to the comment in the EPSG database)
> with the "NAD27 to WGS 84 (41)" EPSG transformation, with an
> advertized accuracy of 1.5m, which combines the CONUS grid and
> us_noaa_cshpgn:

It is only WGS84(TRANSIT) that can reasonably be equated to NAD83(1986),
more or less, and then pretty much all NAD83 are the same.  As I
understand it, the same transit data was used to contrain NAD83(1986)
and WGS84(TRANSIT), and then NAD83 future realizations were aligned
to the previous ones and WGS84 future realizations were aligned to ITRF,
thus memorializing the original TRANSIT mismatch from ITRS goals.

However, when people in 2024 want "WGS84", they almost certainly don't want
WGS84(TRANSIT), but rather "some recent realization which is ~== ITRF at
the cm level".

> I don't think there are any published time-dependent transformations
> for NAD27 to WGS84 or any of its realization.

I don't think so either, but NAD27->NAD83->ITRF has two hops that have
reasonable transformations.  THe first is static-static, on the same
plate, and the second should have a time-dependent transformation
available.