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<p>Unless you are interested in sub-millimetric precision, you
shouldn't care about the difference between GRS80 and WGS84:<br>
</p>
<p><br>
</p>
<p>$ echo 4189881.02 146313.87 4790558.75 | bin/cct -d 4
+proj=pipeline +step +inv +proj=cart +ellps=WGS84 +step
+proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49 +lat_2=44 +x_0=700000
+y_0=6600000 +ellps=GRS80<br>
626830.1633 6878250.0291 0.0017 inf<br>
</p>
<p><br>
</p>
<p>$ echo 4189881.02 146313.87 4790558.75 | bin/cct -d 4
+proj=pipeline +step +inv +proj=cart +ellps=GRS80 +step
+proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49 +lat_2=44 +x_0=700000
+y_0=6600000 +ellps=GRS80<br>
626830.1633 6878250.0292 0.0017 inf<br>
</p>
<p><br>
</p>
<p>And if you care about sub-millimetric precision you shouldn't
attempt at doing that transformation at all, since the "RGF93 v1
to WGS 84 (1)" null-transformation has an advertized accuracy of 1
metre.</p>
<p><br>
</p>
<p>Instead of using EPSG:4978, you'd probably want to use EPSG:4937
(ETRS89 geocentric) to avoid entirely the issues with WGS 84.<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Le 16/10/2024 à 09:26, Christian
DOULIAC a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:0674f806e38e4f88bc261e06bab6fd51@magellium.fr">
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<p>Hello and sorry to spam this subject a bit.<br>
</p>
<p><br>
</p>
<p>I have created a workaround in my code by checking the
"PJ_TYPE" and "<span>proj_crs_promote_to_3D</span>" the CRS if
it is a "<span>PJ_TYPE_GEOGRAPHIC_2D_CRS</span>" or a "<span>PJ_TYPE_PROJECTED_CRS</span>".<br>
</p>
<p><br>
</p>
<p>It works pretty well except for one case : <br>
</p>
<div>Conversion from "EPSG:4978" to "EPSG:2154" <br>
</div>
<div>-In proj 9.2 without promotion the "<span>proj_create_crs_to_crs_from_pj</span>"
function return me a transformation like this :
<br>
<span>+proj=pipeline <br>
+step +inv +proj=cart +ellps=WGS84 <br>
+step +proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49 +lat_2=44
+x_0=700000 +y_0=6600000 +ellps=GRS80</span><br>
</div>
-In proj 9.2 with promotion it gave me this : <br>
<span>+proj=pipeline <br>
</span>
<p><span>+step +inv +proj=cart +ellps=GRS80 <br>
</span></p>
<p><span>+step +proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49
+lat_2=44 +x_0=700000 +y_0=6600000 +ellps=GRS80</span></p>
<p><br>
</p>
<p>The only difference <span>is the change from "+ellps=WGS84"
to "+ellps=GRS80".</span></p>
<div>The 2nd transformation seems to be more accurate because
"EPSG:2154" is based on the GRS80, is it true?
<br>
How can I recreate the past behavior with the C API? (Having <span>"+ellps=WGS84"</span>).
Ideally I would like a code that is not specific for this
case.<br>
<br>
Thank <span>you for taking the time to read</span> me.<br>
</div>
<br>
</div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>De :</b> PROJ
<a class="moz-txt-link-rfc2396E" href="mailto:proj-bounces@lists.osgeo.org"><proj-bounces@lists.osgeo.org></a> de la part de Christian
DOULIAC via PROJ <a class="moz-txt-link-rfc2396E" href="mailto:proj@lists.osgeo.org"><proj@lists.osgeo.org></a><br>
<b>Envoyé :</b> mercredi 2 octobre 2024 14:55:02<br>
<b>À :</b> Even Rouault; <a class="moz-txt-link-abbreviated" href="mailto:proj@lists.osgeo.org">proj@lists.osgeo.org</a><br>
<b>Objet :</b> Re: [PROJ] [Questions] Vertical transformations
advices for PROJ >= 9.1 version</font>
<div> </div>
</div>
<div>
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style="font-size:12pt;color:#000000;font-family:Calibri,Helvetica,sans-serif;"
dir="ltr">
<div>Hello thanks for your quick answer, <br>
<br>
Indeed adding --3d to cs2cs command give me the past
behavior. <br>
The commands that I wrote were only here for the example, to
illustrate that I can recreate the behaviors.<br>
My goal is to be more generic, I would like to be able to do
CRS conversions without knownledges of them at first, using
the PROJ C API. <br>
<br>
I found some advices here
<a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183">https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183</a>
<br>
and here
<a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183">https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183</a>
.<br>
What I understood from theses comments is that promoting CRS
to 3D only make sense for PJ_TYPE_GEOGRAPHIC_2D_CRS and
PJ_TYPE_PROJECTED_CRS types.<br>
I would like to detect theses cases and promoting the CRS
accordingly with the API.<br>
<br>
My questions are multiple : <br>
-first did I understood right? <br>
-Are there others cases (or types) that I should think
about? <br>
-Is looking for the CRS type is enough or should I look for
other information?</div>
</div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>De :</b> Even
Rouault <a class="moz-txt-link-rfc2396E" href="mailto:even.rouault@spatialys.com"><even.rouault@spatialys.com></a><br>
<b>Envoyé :</b> mardi 1 octobre 2024 15:13:11<br>
<b>À :</b> Christian DOULIAC; <a class="moz-txt-link-abbreviated" href="mailto:proj@lists.osgeo.org">proj@lists.osgeo.org</a><br>
<b>Objet :</b> Re: [PROJ] [Questions] Vertical
transformations advices for PROJ >= 9.1 version</font>
<div> </div>
</div>
<div>
<p><br>
</p>
<div class="moz-cite-prefix">Le 01/10/2024 à 11:04, Christian
DOULIAC via PROJ a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:89c8212f2f044a45a02dae850bcf9b32@magellium.fr">
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dir="ltr">
<p dir="auto">Hello, I would like some precision about CRS
conversions.</p>
<p dir="auto">I'm aware that there are some changes about
vertical transformations with PROJ >= 9.1 as stated
here :
<a class="issue-link js-issue-link"
data-error-text="Failed to load title"
data-id="1170903012"
data-permission-text="Title is private"
data-url="https://github.com/OSGeo/PROJ/issues/3119"
data-hovercard-type="pull_request"
data-hovercard-url="/OSGeo/PROJ/pull/3119/hovercard"
href="https://github.com/OSGeo/PROJ/pull/3119"
moz-do-not-send="true">
</a><a href="https://github.com/OSGeo/PROJ/pull/3119"
class="OWAAutoLink moz-txt-link-freetext"
id="LPlnk868644" moz-do-not-send="true">https://github.com/OSGeo/PROJ/pull/3119</a>
<br>
Or here : <a
href="https://proj.org/en/9.4/apps/cs2cs.html#cmdoption-cs2cs-3d"
rel="nofollow" moz-do-not-send="true"
class="moz-txt-link-freetext">
https://proj.org/en/9.4/apps/cs2cs.html#cmdoption-cs2cs-3d</a> "Starting
with PROJ 9.1, both CRS need to be 3D for vertical
transformation to possibly happen."</p>
<p dir="auto"><br>
I tested it and I recreated the behavior of pre 9.1 and
post 9.1 versions :</p>
<p dir="auto">-PROJ 8.2.0 :<br>
echo 0 0 1000 | cs2cs +proj=longlat +datum=WGS84
+geoidgrids=/proj-build/data/for_tests/egm96_15.tif
+vunits=m +no_defs +type=crs +to +proj=longlat
+datum=WGS84 +no_defs +type=crs -f "%.14f"<br>
0.00000000000000 0.00000000000000 1017.16157913208008</p>
<p dir="auto">-PROJ 9.2.0<br>
echo 0 0 1000 | cs2cs +proj=longlat +datum=WGS84
+geoidgrids=/proj-build/data/for_tests/egm96_15.tif
+vunits=m +no_defs +type=crs +to +proj=longlat
+datum=WGS84 +no_defs +type=crs -f "%.14f"<br>
0.00000000000000 0.00000000000000 1000.00000000000000</p>
</div>
</blockquote>
<p>Add --3d to promote the target CRS to 3D.</p>
<p><br>
</p>
<p>Or use: cs2cs -f "%.14f" EPSG:4326+5773 EPSG:4979 (with
lat, long order)<br>
</p>
<span style="white-space: pre-wrap"></span>
<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com"
moz-do-not-send="true">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
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</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.
Butcher of all kinds of standards, open or closed formats. At the end, this is just about bytes.</pre>
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