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<section id="proj-rfc-5-adopt-geotiff-based-grids-for-grids-delivered-with-proj">
<span id="rfc5"></span><h1>PROJ RFC 5: Adopt GeoTIFF-based grids for grids delivered with PROJ<a class="headerlink" href="#proj-rfc-5-adopt-geotiff-based-grids-for-grids-delivered-with-proj" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd"><p>Even Rouault</p>
</dd>
<dt class="field-even">Contact</dt>
<dd class="field-even"><p><a class="reference external" href="mailto:even.rouault%40spatialys.com">even<span>.</span>rouault<span>@</span>spatialys<span>.</span>com</a></p>
</dd>
<dt class="field-odd">Status</dt>
<dd class="field-odd"><p>Adopted</p>
</dd>
<dt class="field-even">Implementation target</dt>
<dd class="field-even"><p>PROJ 7</p>
</dd>
<dt class="field-odd">Last Updated</dt>
<dd class="field-odd"><p>2020-01-28</p>
</dd>
</dl>
<section id="motivation">
<h2>Motivation<a class="headerlink" href="#motivation" title="Permalink to this headline">¶</a></h2>
<p>This RFC is a continuation of <a class="reference internal" href="rfc-4.html#rfc4"><span class="std std-ref">PROJ RFC 4: Remote access to grids and GeoTIFF grids</span></a>. With RFC4, PROJ can, upon request
of the user, download grids from a CDN in a progressive way. There is also API,
such as <code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">proj_download_file()</span></code> to be able to download a GeoTIFF grid in
the user writable directory. The content of the CDN at <a class="reference external" href="https://cdn.proj.org">https://cdn.proj.org</a>
is <a class="reference external" href="https://github.com/OSGeo/PROJ-data">https://github.com/OSGeo/PROJ-data</a> , which has the same content
as <a class="reference external" href="https://github.com/OSGeo/proj-datumgrid">https://github.com/OSGeo/proj-datumgrid</a> converted in GeoTIFF files. In the
current state, we could have a somewhat inconsistency between users relying on
the proj-datumgrid, proj-datumgrid-[world,northamerica,oceania,europe] packages
of mostly NTv2 and GTX files, and what is shipped through the CDN. Maintaining
two repositories is also a maintenance burden in the long term.</p>
<p>It is thus desirable to have a single source of truth, and we propose it to be
based on the GeoTIFF grids.</p>
</section>
<section id="summary-of-work-planned-by-this-rfc-and-related-decisions">
<h2>Summary of work planned by this RFC and related decisions<a class="headerlink" href="#summary-of-work-planned-by-this-rfc-and-related-decisions" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p><a class="reference external" href="https://github.com/OSGeo/PROJ-data/">https://github.com/OSGeo/PROJ-data/</a> will be used, starting with
PROJ 7.0, to create “static” grid packages.</p></li>
<li><p>For now, a single package of, mostly GeoTIFF grids (a few text files for
PROJ init style files, as well as a few edge cases for deformation models where
grids have not been converted), will be delivered.
Its size at the time of writing is 486 MB (compared to 1.5 GB of uncompressed
NTv2 + GTX content, compressed to ~ 700 MB currently)</p></li>
<li><p>The content of this archive will be flat, i.e. no subdirectories</p></li>
<li><p>Each file will be named according to the following pattern
<code class="docutils literal notranslate"><span class="pre">${agency_name}_${filename}[.ext]</span></code>. For example fr_ign_ntf_r93.tif
This convention should allow packagers, if the need arise, to be able to
split the monolithic package in smaller ones, based on criterion related to
the country.</p>
<p>The agency name is the one you can see from the directory names at
<a class="reference external" href="https://github.com/OSGeo/PROJ-data/">https://github.com/OSGeo/PROJ-data/</a>.
<code class="docutils literal notranslate"><span class="pre">${agency_name}</span></code> itself is structure like <code class="docutils literal notranslate"><span class="pre">${two_letter_country_code_of_agency_nationality}_${some_abbreviation}</span></code>
(with the exception of eur_nkg, for the Nordic Geodetic Commission which
isn’t affiliated to a single country but to some European countries, and
follows the general scheme)</p>
</li>
<li><p><a class="reference external" href="https://github.com/OSGeo/proj-datumgrid">https://github.com/OSGeo/proj-datumgrid</a> and related packages will only be
maintained during the remaining lifetime of PROJ 6.x. After that, the
repository will no longer receive any update and will be put in archiving
state (see <a class="reference external" href="https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-archiving-repositories">https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-archiving-repositories</a>)</p></li>
<li><p>PROJ database <code class="docutils literal notranslate"><span class="pre">grid_alternatives</span></code> table will be updated to point to the new
TIFF filenames. It will also maintain the old names as used by current
proj-datumgrid packages to be able to provide backward compatibility when
a PROJ string refers to a grid by its previous name.</p></li>
<li><p>Upon adoption of this RFC, new grids referenced by PROJ database will only
point to GeoTIFF grid names.</p></li>
<li><p>Related to the above point, if a PROJ string refers to a grid name, let’s
say foo.gsb. This grid will first be looked for in all the relevant locations
under this name. If no match is found, then a lookup in the
<code class="docutils literal notranslate"><span class="pre">grid_alternatives</span></code> table will be done to retrieve the potential new name
(GeoTIFF file), and if there’s such match, a new look-up in the file system
will be done with the name of this GeoTIFF file.</p></li>
<li><p>The <code class="docutils literal notranslate"><span class="pre">package_name</span></code> column of grid_alternatives will no longer be filled.
And <code class="docutils literal notranslate"><span class="pre">url</span></code> will be filled with the direct URL to the grid in the CDN, for
example: <a class="reference external" href="https://cdn.proj.org/fr_ign_ntf_r93.tif">https://cdn.proj.org/fr_ign_ntf_r93.tif</a></p></li>
<li><p>The Python scripts to convert grids (NTv2, GTX) to GeoTIFF currently available at
<a class="reference external" href="https://github.com/rouault/sample_proj_gtiff_grids/">https://github.com/rouault/sample_proj_gtiff_grids/</a> will be moved to a
grid_tools/ subdirectories of <a class="reference external" href="https://github.com/OSGeo/PROJ-data/">https://github.com/OSGeo/PROJ-data/</a>
Documentation for those utilities will be added to PROJ documentation.</p></li>
<li><p>Obviously, all the above assumes PROJ builds to have libtiff enabled.
Non-libtiff builds are not considered as nominal PROJ builds (if a PROJ master
build is attempted and libtiff is not detected, it fails. The user has to
explicitly ask to disable TIFF support), and users deciding to go through that
route will have to deal with the consequences (that is that
grid-based transformations generated by PROJ will likely be non working)</p></li>
</ul>
</section>
<section id="backward-compatibility">
<h2>Backward compatibility<a class="headerlink" href="#backward-compatibility" title="Permalink to this headline">¶</a></h2>
<p>This change is considered to be <em>mostly</em> backward compatible. There might be
impacts for software using <code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">proj_coordoperation_get_grid_used()</span></code> and
assuming that the url returned is one of the proj-datumgrid-xxx files at
<a class="reference external" href="https://download.osgeo.org">https://download.osgeo.org</a>. As mentioned in
<a class="reference external" href="https://lists.osgeo.org/pipermail/proj/2020-January/009274.html">https://lists.osgeo.org/pipermail/proj/2020-January/009274.html</a> , this
assumption was not completely bullet-proof either.
There will be impacts on software checking the value of PROJ pipeline strings
resulting <code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">proj_create_crs_to_crs()</span></code>. The new grid names will now
be returned (the most impacted software will likely be PROJ’s own test suite)</p>
<p>Although discouraged, people not using the new proj-datumgrid-geotiff-XXX.zip
archives, should still be able to use the old archives made of NTv2/GTX files,
at least as long as the PROJ database does not only point to a GeoTIFF grid.
So this might be a short-term partly working solution, but at time goes, it
will become increasingly non-working. The nominal combination will be
PROJ 7.0 + proj-datumgrid-geotiff-1.0.zip</p>
</section>
<section id="testing">
<h2>Testing<a class="headerlink" href="#testing" title="Permalink to this headline">¶</a></h2>
<p>PROJ test suite will have to be adapted for the new TIFF based filenames.</p>
<p>Mechanism to auto-promote existing NTv2/GTX names to TIFF ones will be exercised.</p>
</section>
<section id="proposed-implementation">
<h2>Proposed implementation<a class="headerlink" href="#proposed-implementation" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="https://github.com/OSGeo/PROJ/pull/1891">https://github.com/OSGeo/PROJ/pull/1891</a> and <a class="reference external" href="https://github.com/OSGeo/PROJ-data/pull/5">https://github.com/OSGeo/PROJ-data/pull/5</a></p>
</section>
<section id="adoption-status">
<h2>Adoption status<a class="headerlink" href="#adoption-status" title="Permalink to this headline">¶</a></h2>
<p>The RFC was adopted on 2020-01-28 with +1’s from the following PSC members</p>
<ul class="simple">
<li><p>Kristian Evers</p></li>
<li><p>Even Rouault</p></li>
<li><p>Thomas Knudsen</p></li>
<li><p>Howard Butler</p></li>
<li><p>Kurt Schwehr</p></li>
</ul>
</section>
</section>
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