<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>Shawn,</p>
<p>The results depends on which PROJ grids are available on your
system (or if you allowed network access).</p>
<p>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:<br>
</p>
<p>$ 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<br>
32.80423938 -117.25978145 0.00000000 inf</p>
<p>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:</p>
<p>$ echo 32.804191 -117.258911 | cs2cs -d 8 NAD27 WGS84<br>
32.80423884 -117.25976448 0.00000000<br>
</p>
<p>or explicitly with the pipeline show by "projinfo NAD27 WGS84
--bbox -118,32,-117,33 --spatial-test intersects --single-line":<br>
</p>
<p>$ 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<br>
32.80423884 -117.25976448 0.00000000 inf</p>
<p>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:</p>
<p>$ echo 32.804191 -117.258911 | PROJ_NETWORK=ON cs2cs -d 8 NAD27
WGS84<br>
32.80424065 -117.25978105 0.00000000</p>
<p>or explicitly with the pipeline show by "projinfo NAD27 WGS84
--bbox -118,32,-117,33 --spatial-test intersects --single-line":</p>
<p>$ 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<br>
32.80424065 -117.25978105 0.00000000 inf<br>
</p>
<p>==> 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
<a class="moz-txt-link-freetext" href="https://www.youtube.com/watch?v=M2ck3cAGvhg">https://www.youtube.com/watch?v=M2ck3cAGvhg</a>). I don't think there
are any published time-dependent transformations for NAD27 to
WGS84 or any of its realization.<br>
</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 24/08/2024 à 03:14, Fox, Shawn D
(US) via gdal-dev a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:20240824011409.B40636142C92@lists.osgeo.org">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta name="Generator"
content="Microsoft Word 15 (filtered medium)">
<style>@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
font-size:11.0pt;
font-family:"Calibri",sans-serif;
mso-ligatures:standardcontextual;}a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}.MsoChpDefault
{mso-style-type:export-only;}div.WordSection1
{page:WordSection1;}</style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<div class="WordSection1">
<p class="MsoNormal">Hello GDAL users.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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. <o:p></o:p></p>
<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>GDAL 1.10.0 results<o:p></o:p></b></p>
<p class="MsoNormal">./gdaltransform -s_srs NAD27 -t_srs WGS84<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-117.258911 32.804191<o:p></o:p></p>
<p class="MsoNormal">-117.2597<span style="color:red">8144845</span>
32.80423<span style="color:red">93819884</span>
1.32247805595398e-07<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>GDAL 3.4.2 results<o:p></o:p></b></p>
<p class="MsoNormal">./gdaltransform -s_srs NAD27 -t_srs WGS84<o:p></o:p></p>
<p class="MsoNormal">-117.258911 32.804191<o:p></o:p></p>
<p class="MsoNormal">-117.2597<span style="color:red">64476223</span>
32.80423<span style="color:red">88409449</span> 0<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have read that the support for specifying
epochs related to realizations is not available until GDAL
3.8.0.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Here are some references I’ve read so far.<o:p></o:p></p>
<p class="MsoNormal"><a
href="https://ascelibrary.org/doi/10.1061/%28ASCE%29SU.1943-5428.0000389"
moz-do-not-send="true">Transforming between WGS84
Realizations | Journal of Surveying Engineering | Vol 148,
No 2 (ascelibrary.org)</a><o:p></o:p></p>
<p class="MsoNormal"><a
href="https://gdal.org/development/rfc/rfc81_coordinate_epoch.html"
moz-do-not-send="true">RFC 81: Support for coordinate epochs
in geospatial formats — GDAL documentation</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black;mso-ligatures:none">Shawn
Fox<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black;mso-ligatures:none">San
Diego, CA</span><span
style="font-size:8.0pt;mso-ligatures:none">
</span><span style="mso-ligatures:none"><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
gdal-dev mailing list
<a class="moz-txt-link-abbreviated" href="mailto:gdal-dev@lists.osgeo.org">gdal-dev@lists.osgeo.org</a>
<a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/gdal-dev">https://lists.osgeo.org/mailman/listinfo/gdal-dev</a>
</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
</body>
</html>