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

[Fox, Shawn D (US)]

Hello GDAL users.

I have noticed that from GDAL 1.10 - GDAL 3.1.0 that gdaltransform is producing different results for NAD27 GCS to WGS84 GCS transformations.  The differences in longitude in the output is more significant.  Having tested the differences in feet between the points output from each version I can see a difference of ~5 feet primarily due to the change in the longitude value.
Would anyone look at the following results and share your thoughts about why the result of this transformation is different in a newer version?  Could this be related to the concept of WGS 84 realization being different in newer versions of GDAL?  I am also linking GDAL 3.4.2 with PROJ 8.1.1 at this time.  These results were produced with GDAL 3.4.2, but the same results occur with GDAL 3.1.0 as well.  GDAL 3.4.2 is what I am currently trying to upgrade to.  The results from the C++ programming API are similar in difference.  I just used gdaltransform for the purposes of this message because it was a more convenient way of producing something that others might be able to reproduce.

GDAL 1.10.0 results
./gdaltransform -s_srs NAD27 -t_srs WGS84

-117.258911 32.804191
-117.25978144845 32.8042393819884 1.32247805595398e-07

GDAL 3.4.2 results
./gdaltransform -s_srs NAD27 -t_srs WGS84
-117.258911 32.804191
-117.259764476223 32.8042388409449 0

I have read that the support for specifying epochs related to realizations is not available until GDAL 3.8.0.

Here are some references I've read so far.
Transforming between WGS84 Realizations | Journal of Surveying Engineering | Vol 148, No 2 (ascelibrary.org)<https://ascelibrary.org/doi/10.1061/%28ASCE%29SU.1943-5428.0000389>
RFC 81: Support for coordinate epochs in geospatial formats - GDAL documentation<https://gdal.org/development/rfc/rfc81_coordinate_epoch.html>

Thanks,
Shawn Fox
San Diego, CA

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