Shawn,
The results depends on which PROJ grids are available on your system (or
if you allowed network access).
1) The result you get with GDAL 1.10 can be reproduced with modern PROJ
versions by allowing (and restricting) to using the CONUS /
us_noaa_conus.tif grid which corresponds to the "NAD27 to WGS 84 (79)"
EPSG transformation, with an advertized accuracy of 5.0 m:
$ echo 32.804191 -117.258911 0 | PROJ_NETWORK=ON cct -d 8 +proj=pipeline
+step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg
+xy_out=rad +step +proj=hgridshift +grids=us_noaa_conus.tif +step
+proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
32.80423938 -117.25978145 0.00000000 inf
2) The result you get with GDAL 3.4.2 / PROJ 8.1.1 can be actually
reproduced by using only the "NAD27 to WGS 84 (6)" EPSG Helmert
transformation, with an advertized accuracy of 7.0 m, which is the most
precise one in the absence of any grid:
$ echo 32.804191 -117.258911 | cs2cs -d 8 NAD27 WGS84
32.80423884 -117.25976448 0.00000000
or explicitly with the pipeline show by "projinfo NAD27 WGS84 --bbox
-118,32,-117,33 --spatial-test intersects --single-line":
$ echo 32.804191 -117.258911 0 | cct -d 8 +proj=pipeline +step
+proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad
+step +proj=push +v_3 +step +proj=cart +ellps=clrk66 +step +proj=helmert
+x=-8 +y=159 +z=175 +step +inv +proj=cart +ellps=WGS84 +step +proj=pop
+v_3 +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap
+order=2,1
32.80423884 -117.25976448 0.00000000 inf
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:
$ echo 32.804191 -117.258911 | PROJ_NETWORK=ON cs2cs -d 8 NAD27 WGS84
32.80424065 -117.25978105 0.00000000
or explicitly with the pipeline show by "projinfo NAD27 WGS84 --bbox
-118,32,-117,33 --spatial-test intersects --single-line":
$ echo 32.804191 -117.258911 0 | PROJ_NETWORK=ON cct -d 8 +proj=pipeline
+step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg
+xy_out=rad +step +proj=hgridshift +grids=us_noaa_conus.tif +step
+proj=hgridshift +grids=us_noaa_cshpgn.tif +step +proj=unitconvert
+xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
32.80424065 -117.25978105 0.00000000 inf
==> All those results are valid. Some are more precise than others,
under certain assumptions... But basically speaking about WGS 84
introduces an intrinsic inaccuracy of about 2 meters (cf
https://www.youtube.com/watch?v=M2ck3cAGvhg). I don't think there are
any published time-dependent transformations for NAD27 to WGS84 or any
of its realization.
Even
Le 24/08/2024 à 03:14, Fox, Shawn D (US) via gdal-dev a écrit :
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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