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<p>Floris,</p>
<p>I believe we'll have to support indefinitely PROJ.4 style CRS
strings since there are "everywhere", at least from a legacy point
of view. They're for example included in some geospatial datasets
to define their CRS (netCDF, other formats handled by GDAL). They
have clearly some limitations, but are sufficient for many use
cases.<br>
</p>
<p>I'd suggest looking at PROJJSON
(<a class="moz-txt-link-freetext" href="https://proj.org/specifications/projjson.html">https://proj.org/specifications/projjson.html</a>), which is a
JSON-ified version of WKT, with the same expressiveness but a less
awkward syntax. It is not yet standardized, but there's an effort
just starting to make it a OGC standard/community standard.<br>
</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 28/06/2021 à 16:47, Floris
Vanderhaeghe via PROJ a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:5702f5c0-0b0a-2f7b-7f09-44575fc5898c@inbo.be"> Hi,<br>
<p>Below are some questions about the usage of 'PROJ strings' (as
opposed to using WKT or AUTHORITY:CODE) as an option to declare
the CRS of coordinates. The context is geospatial work in R with
PROJ >= 6, the fact that users <i>can</i> still input PROJ
strings to declare an object's CRS, and the question what to
advise to users. We already had some discussions on this in the
R-spatial community, which lead me to posing some questions
here.<br>
</p>
<p>After reading through several PROJ documentation pages, and
observing output from 'projinfo':</p>
<ol>
<li>It is clear that currently, support for representing a CRS
by using a PROJ string is only partial, especially because the
datum can often not be specified. ==> <b>QUESTION</b>: <u>Is
specifying a CRS with a PROJ string going to receive
continued support, or will it eventually be dropped? And
related with that, </u><u>[<b>QUESTION</b>:] should we
still advertise and use it to represent a CRS?</u> The
current state may appear somewhat unclear:</li>
<ul>
<li>There is support for '+type = crs' (to declare a CRS)</li>
<ul>
<li>e.g. projinfo does accept a PROJ string as input to
represent a CRS, by inclusion of '+type=crs'<br>
</li>
</ul>
<li>There is (still) support for '+towgs84' (to declare a
BOUNDCRS, hence not defining the datum directly)</li>
<ul>
<li>Cf. mention of '+towgs84' in the context of a pipeline,
in <a class="moz-txt-link-freetext"
href="https://proj.org/operations/operations_computation.html#when-the-source-or-target-crs-is-a-boundcrs"
moz-do-not-send="true">https://proj.org/operations/operations_computation.html#when-the-source-or-target-crs-is-a-boundcrs</a><br>
</li>
<li>'+towgs84' can be still used in a PROJ string that
represents just a CRS (combining '+towgs84=' and
'+type=crs')</li>
</ul>
<li>However, PROJ strings are now generally described in the
documentation as representing a coordinate operation,
including datum <i>shifts</i> when using a pipeline string.
That is, the pipeline itself does not define input and
output datums, but may contain a transformation step to
perform a datum shift.</li>
<li>In several places the <i>deprecation</i> of '+towgs84' is
referred --> <b>QUESTION</b>: <u>will '+towgs84' be
dropped eventually? </u></li>
<li>Although using '+type = crs', the datum cannot be
specified: it is WGS84 when no '+ellps' is provided, and
else 'unknown'. But with an unknown datum, the CRS is
incomplete. <b>QUESTION</b>: <u>Will</u><u> '+type = crs'
get continued support, or is it intended to eventually end
support for '+type=crs'?</u></li>
<li>A comment in the second code chunk under <a
class="moz-txt-link-freetext"
href="https://proj.org/development/migration.html#code-example"
moz-do-not-send="true">https://proj.org/development/migration.html#code-example</a>
clearly disrecommends PROJ strings to define a CRS (as
opposed to WKT or AUTHORITY:CODE), but generally this advice
seems rather sparse in the documentation. In fact, the
still-supported use of a PROJ string to define a CRS is only
touched under <a
href="https://proj.org/development/reference/functions.html#c.proj_create"
moz-do-not-send="true">proj_create()</a> and discouraged.</li>
</ul>
<li><i>If PROJ strings should not be used to define a CRS, then:
</i>[<b>QUESTION</b>:] <u>is it possible, with currently
advised methods, to define a </u><u><i>custom</i></u><u>
CRS in a relatively short way</u>, i.e. not having to write
out a full WKT? Perhaps having the possibility to refer an
authority CRS (just using its code or name), and a way to
alter specific (WKT?) parameters, could work, although I don't
know whether specification of the second part can be
standardised easily.</li>
</ol>
<p>Thanks in advance! I am not an experienced PROJ user.<br>
</p>
<p>With regards,</p>
<p>Floris</p>
<br>
<div class="moz-signature">
<title>mailhandtekening</title>
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<p><b><span lang="EN-US">Floris Vanderhaeghe</span></b><br>
</p>
<p><span>Flemish Government<br>
RESEARCH INSTITUTE FOR NATURE AND FOREST<br>
</span><span>Team Biometry, Methodology and
Quality Assurance<br>
</span><span>Havenlaan 88 bus 73, 1000
Brussels<br>
Belgium</span><span><br>
<a href="mailto:floris.vanderhaeghe@inbo.be"
moz-do-not-send="true">floris.vanderhaeghe@inbo.be</a><br>
</span><a href="http://www.inbo.be/"
title="http://www.inbo.be/
blocked::http://www.inbo.be/"
target="_blank" moz-do-not-send="true">www.inbo.be</a></p>
<div><span>///////////////////////////////////////////////////////////////////////////////////////////</span></div>
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