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<div class="moz-cite-prefix">Le 29/06/2021 à 17:27, Floris
Vanderhaeghe via PROJ a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
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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>
</ol>
</blockquote>
Yes<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
<li>Also custom CRSs are ideally specified as <a
href="https://docs.opengeospatial.org/is/18-010r7/18-010r7.html"
moz-do-not-send="true">WKT2</a>, or more conveniently as
(currently not yet official) <a
href="https://proj.org/specifications/projjson.html"
moz-do-not-send="true">PROJJSON</a>, which follows the WKT2
structure.</li>
</ol>
</blockquote>
Yes<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
<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').</li>
</ol>
</blockquote>
Yes<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
<li> 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>
</ol>
</blockquote>
No. For EPSG, EPSG:6326 is the datum ensemble WGS 84, not the latest
realization. We have indeed discussed a number of times on this list
if we should consider it as the latest realization in practice, but
haven't decided so yet.<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
<li>For backward compatibility reasons, PROJ strings with
'+towgs84' or '+datum', although deprecated, will remain
supported by PROJ and GDAL.<br>
</li>
</ol>
</blockquote>
Yes<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
<li> <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>
</blockquote>
Yes<br>
<blockquote type="cite"
cite="mid:bc873f3f-ff04-6cce-167a-1b23b0117bec@inbo.be">
<ol>
</ol>
<p>With regards</p>
<p>Floris<br>
</p>
<br>
<br>
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<p><font color="#666666"><b
style="font-size:12.8px"><span
style="font-size:9pt;font-family:Verdana,sans-serif"
lang="EN-US">Floris Vanderhaeghe</span></b><br>
</font></p>
<p><font color="#666666"><span
style="font-size:9pt;font-family:Verdana,sans-serif">Flemish
Government<br>
RESEARCH INSTITUTE FOR NATURE AND FOREST<br>
</span><font face="Verdana, sans-serif"><span
style="font-size:12px">Team Biometry,
Methodology and Quality Assurance<br>
</span></font><span
style="font-family:Verdana,sans-serif;font-size:9pt">Havenlaan
88 bus 73, 1000 Brussels<br>
Belgium<br>
</span><span
style="font-size:9pt;font-family:Verdana,sans-serif"><a
href="mailto:floris.vanderhaeghe@inbo.be" style=""
moz-do-not-send="true"><font
color="#666666">floris.vanderhaeghe@inbo.be</font></a><br>
</span><a href="http://www.inbo.be/"
title="http://www.inbo.be/
blocked::http://www.inbo.be/"
target="_blank"
style="font-family:Verdana,sans-serif;font-size:9pt"
moz-do-not-send="true"><font
color="#666666">www.inbo.be</font></a></font></p>
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style="font-family:verdana,sans-serif"><font
size="1" color="#666666">///////////////////////////////////////////////////////////////////////////////////////////</font></span></div>
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