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<div class="moz-cite-prefix">Le 29/06/2021 à 14:59, Floris
Vanderhaeghe via PROJ a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:2febc290-1039-67dc-3137-0ff5abbcdc25@inbo.be">So I guess
'+towgs84' is also a feature that will remain?? (I ask because
'+towgs84' and '+datum' are considered deprecated.)<br>
</blockquote>
<p>+towgs84 is deprecated in the sense that using WGS84 as a pivot
datum may add accuracy loss in transformations.</p>
<p>+datum is fine but there is only a small list of datums possible.<br>
</p>
<blockquote type="cite"
cite="mid:2febc290-1039-67dc-3137-0ff5abbcdc25@inbo.be">
<p>Further, after comparing projinfo (in PROJ 7.2.1) and
gdalsrsinfo (in GDAL 3.2.1), it can be seen that:</p>
<ul>
<li>PROJ needs '+type=crs' in an inputted PROJ string to define
a CRS, otherwise it is seen as a coordinate operation (as
expected from most PROJ documentation)<br>
</li>
<li>GDAL seems to always interpret a PROJ string as a CRS, also
without '+type=crs'. This seems in contradiction with PROJ
(but maybe has its reasons)</li>
</ul>
</blockquote>
Yes, the GDAL behaviour is for backward compatibility with GDAL <
3<br>
<blockquote type="cite"
cite="mid:2febc290-1039-67dc-3137-0ff5abbcdc25@inbo.be">
<ul>
<li>both projinfo and gdalsrsinfo currently honour '+towgs84' in
the input (returning a BOUNDCRS), and repeat the exact same
PROJ string in the output (except that GDAL drops
'+type=crs'). However '+towgs84' (as well as '+datum') is not
in the returned PROJ string if the input is AUTHORITY:CODE, so
such PROJ strings are partial CRS representations.<br>
</li>
</ul>
</blockquote>
<p>It really depends on the AUTHORITY:CODE. If there's only one
transformation know from the datum of the CRS to WGS 84, then it
will be used for +towgs84. e.g:<br>
</p>
<p>$ projinfo EPSG:4171 -o PROJ -q<br>
+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs
+type=crs<br>
If there are several choices possible, no +towgs84 is emitted</p>
--
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