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<p>Thank you Even!</p>
<p>To R users, would this be appropriate advice regarding PROJ
strings? - please comment or edit where needed:</p>
<ol>
<li>Using a PROJ string to specify a CRS is <a
moz-do-not-send="true"
href="https://proj.org/development/reference/functions.html#c.proj_create">discouraged</a>
and currently not much documented by PROJ (PROJ strings are used
for coordinate operations instead). At least for registered
CRSs, always use AUTHORITY:CODE or <a moz-do-not-send="true"
href="https://docs.opengeospatial.org/is/18-010r7/18-010r7.html">WKT2</a>,
not PROJ strings.</li>
<li>Also custom CRSs are ideally specified as <a
href="https://docs.opengeospatial.org/is/18-010r7/18-010r7.html">WKT2</a>,
or more conveniently as (currently not yet official) <a
href="https://proj.org/specifications/projjson.html">PROJJSON</a>,
which follows the WKT2 structure.</li>
<li>When specifying a custom CRS with a PROJ string, the WGS84
ensemble datum (<a moz-do-not-send="true"
href="https://epsg.org/datum_6326/World-Geodetic-System-1984-ensemble.html">EPSG:6326</a>)
will automatically be adopted if no '+ellps' is specified (and
with '+ellps', the datum is considered as 'unknown'). Note that
EPSG:6326 always refers to the latest WGS84 realization, not to
a fixed one. If these conditions are fine for your usecase, then
there's no problem.</li>
<li>For backward compatibility reasons, PROJ strings with
'+towgs84' or '+datum', although deprecated, will remain
supported by PROJ and GDAL.<br>
</li>
<li>When specifying a custom CRS that doesn't use the WGS84, NAD27
or NAD83 datum (which can be specified with '+datum'), it is
better to use <a moz-do-not-send="true"
href="https://docs.opengeospatial.org/is/18-010r7/18-010r7.html">WKT2</a>
or <a moz-do-not-send="true"
href="https://proj.org/specifications/projjson.html">PROJJSON</a>
and properly refer the geodetic datum. A typically less accurate
alternative is defining as a BOUNDCRS by specifying '+towgs84'
transformation parameters.</li>
</ol>
<p>With regards</p>
<p>Floris<br>
</p>
<br>
<br>
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<p><font color="#666666"><b
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lang="EN-US">Floris Vanderhaeghe</span></b><br>
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<p><font color="#666666"><span
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RESEARCH INSTITUTE FOR NATURE AND FOREST<br>
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