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PROJ</a><ul> +<li class="toctree-l4"><a class="reference internal" href="#motivation">Motivation</a></li> +<li class="toctree-l4"><a class="reference internal" href="#summary-of-work-planned-by-this-rfc-and-related-decisions">Summary of work planned by this RFC and related decisions</a></li> +<li class="toctree-l4"><a class="reference internal" href="#backward-compatibility">Backward compatibility</a></li> +<li class="toctree-l4"><a class="reference internal" href="#testing">Testing</a></li> +<li class="toctree-l4"><a class="reference internal" href="#proposed-implementation">Proposed implementation</a></li> +<li class="toctree-l4"><a class="reference internal" href="#adoption-status">Adoption status</a></li> +</ul> +</li> +<li class="toctree-l3"><a class="reference internal" href="rfc-6.html">PROJ RFC 6: Triangulation-based transformations</a></li> +<li class="toctree-l3"><a class="reference internal" href="rfc-7.html">PROJ RFC 7: Drop Autotools, maintain CMake</a></li> +</ul> +</li> 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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> + + + </div> + </div> + <footer><div class="rst-footer-buttons" role="navigation" aria-label="Footer"> + <a href="rfc-4.html" class="btn btn-neutral float-left" title="PROJ RFC 4: Remote access to grids and GeoTIFF grids" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left" aria-hidden="true"></span> Previous</a> + <a href="rfc-6.html" class="btn btn-neutral float-right" title="PROJ RFC 6: Triangulation-based transformations" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right" aria-hidden="true"></span></a> + </div> + + <hr/> + + <div role="contentinfo"> + <p>© Copyright 1983-2022. + <span class="lastupdated">Last updated on 22 Mar 2022. + </span></p> + </div> + + Built with <a href="https://www.sphinx-doc.org/">Sphinx</a> using a + <a href="https://github.com/readthedocs/sphinx_rtd_theme">theme</a> + provided by <a href="https://readthedocs.org">Read the Docs</a>. + + +</footer> + </div> + </div> + </section> + </div> + <script> + jQuery(function () { + SphinxRtdTheme.Navigation.enable(true); + }); + </script> + +</body> +</html>
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