update with results of commit https://github.com/OSGeo/PROJ/commit/53c07a8bd211b7aee4bc07a9c6726005504b7181gh-pages

author: PROJ deploybot <proj.bot@proj.bot> 2022-03-22 20:00:06 +0000
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+<li class="toctree-l2 current"><a class="current reference internal" href="#">The pipeline operator</a><ul>
+<li class="toctree-l3"><a class="reference internal" href="#rules-for-pipelines">Rules for pipelines</a></li>
+<li class="toctree-l3"><a class="reference internal" href="#parameters">Parameters</a><ul>
+<li class="toctree-l4"><a class="reference internal" href="#required">Required</a></li>
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+           <div itemprop="articleBody">
+             
+  <section id="the-pipeline-operator">
+<span id="pipeline"></span><h1>The pipeline operator<a class="headerlink" href="#the-pipeline-operator" title="Permalink to this headline">¶</a></h1>
+<div class="versionadded">
+<p><span class="versionmodified added">New in version 5.0.0.</span></p>
+</div>
+<p>Construct complex operations by daisy-chaining operations in a sequential pipeline.</p>
+<table class="docutils align-default">
+<colgroup>
+<col style="width: 20%" />
+<col style="width: 80%" />
+</colgroup>
+<tbody>
+<tr class="row-odd"><td><p><strong>Alias</strong></p></td>
+<td><p>pipeline</p></td>
+</tr>
+<tr class="row-even"><td><p><strong>Domain</strong></p></td>
+<td><p>2D, 3D and 4D</p></td>
+</tr>
+<tr class="row-odd"><td><p><strong>Input type</strong></p></td>
+<td><p>Any</p></td>
+</tr>
+<tr class="row-even"><td><p><strong>Output type</strong></p></td>
+<td><p>Any</p></td>
+</tr>
+</tbody>
+</table>
+<div class="admonition note">
+<p class="admonition-title">Note</p>
+<p>See the section on <a class="reference internal" href="../usage/transformation.html#transformation"><span class="std std-ref">Geodetic transformation</span></a> for a more thorough introduction
+to the concept of transformation pipelines in PROJ.</p>
+</div>
+<p>With the pipeline operation it is possible to perform several operations after each
+other on the same input data. This feature makes it possible to create transformations
+that are made up of more than one operation, e.g. performing a datum shift and then
+applying a suitable map projection. Theoretically any transformation between two
+coordinate reference systems is possible to perform using the pipeline operation,
+provided that the necessary coordinate operations in each step is available in PROJ.</p>
+<p>A pipeline is made up of a number of steps, with each step being a coordinate operation
+in itself. By connecting these individual steps sequentially we end up with a concatenated
+coordinate operation. An example of this is a transformation from geodetic coordinates
+on the GRS80 ellipsoid to a projected system where the east-west and north-east axes has
+been swapped:</p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline +ellps=GRS80 +step +proj=merc +step +proj=axisswap +order=2,1
+</pre></div>
+</div>
+<p>Here the first step is applying the <a class="reference internal" href="projections/merc.html#merc"><span class="std std-ref">Mercator</span></a> projection and the second step is
+applying the <a class="reference internal" href="conversions/axisswap.html#axisswap"><span class="std std-ref">Axis swap</span></a> conversion. Note that the <cite>+ellps=GRS80</cite> is specified
+before the first occurrence of <cite>+step</cite>. This means that the GRS80 ellipsoid is used
+in both steps, since any parameter stated before the first occurrence of <cite>+step</cite> is
+treated as a global parameter and is transferred to each individual steps.</p>
+<section id="rules-for-pipelines">
+<h2>Rules for pipelines<a class="headerlink" href="#rules-for-pipelines" title="Permalink to this headline">¶</a></h2>
+<p><strong>1. Pipelines must consist of at least one step.</strong></p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
+</pre></div>
+</div>
+<p>Will result in an error.</p>
+<p><strong>2. Pipelines can only be nested if the nested pipeline is defined in an init file.</strong></p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
++step +proj=pipeline +step +proj=merc +step +proj=axisswap +order=2,1
++step +proj=unitconvert +xy_in=m +xy_out=us-ft
+</pre></div>
+</div>
+<p>Results in an error, while</p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
++step +init=predefined_pipelines:projectandswap
++step +proj=unitconvert +xy_in=m +xy_out=us-ft
+</pre></div>
+</div>
+<p>does not.</p>
+<p><strong>3. Pipelines without a forward path can’t be constructed.</strong></p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline +step +inv +proj=urm5
+</pre></div>
+</div>
+<p>Will result in an error since <a class="reference internal" href="projections/urm5.html#urm5"><span class="std std-ref">Urmaev V</span></a> does not have an inverse operation defined.</p>
+<p id="global-pipeline-parameter"><strong>4. Parameters added before the first `+step` are global and will be applied to all steps.</strong></p>
+<p>In the following the GRS80 ellipsoid will be applied to all steps.</p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline +ellps=GRS80
++step +proj=cart
++step +proj=helmert +x=10 +y=3 +z=1
++step +proj=cart +inv
++step +proj=merc
+</pre></div>
+</div>
+<p><strong>5. Units of operations must match between steps.</strong></p>
+<div class="versionadded">
+<p><span class="versionmodified added">New in version 5.1.0.</span></p>
+</div>
+<p>The output units of step <em>n</em> must match the expected input unit of step <em>n+1</em>. E.g., you can’t
+pass an operation that outputs projected coordinates to an operation that expects angular units
+(degrees). An example of such a unit mismatch is displayed below.</p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
++step +proj=merc  # Mercator outputs projected coordinates
++step +proj=robin # The Robinson projection expects angular input
+</pre></div>
+</div>
+</section>
+<section id="parameters">
+<h2>Parameters<a class="headerlink" href="#parameters" title="Permalink to this headline">¶</a></h2>
+<section id="required">
+<h3>Required<a class="headerlink" href="#required" title="Permalink to this headline">¶</a></h3>
+<dl class="std option">
+<dt class="sig sig-object std" id="cmdoption-arg-step">
+<span class="sig-name descname"><span class="pre">+step</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-step" title="Permalink to this definition">¶</a></dt>
+<dd><p>Separate each step in a pipeline.</p>
+</dd></dl>
+
+</section>
+<section id="optional">
+<h3>Optional<a class="headerlink" href="#optional" title="Permalink to this headline">¶</a></h3>
+<dl class="std option">
+<dt class="sig sig-object std" id="cmdoption-arg-inv">
+<span class="sig-name descname"><span class="pre">+inv</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-inv" title="Permalink to this definition">¶</a></dt>
+<dd><p>Invert a step in a pipeline.</p>
+</dd></dl>
+
+<dl class="std option">
+<dt class="sig sig-object std" id="cmdoption-arg-omit_fwd">
+<span id="cmdoption-arg-omit-fwd"></span><span class="sig-name descname"><span class="pre">+omit_fwd</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-omit_fwd" title="Permalink to this definition">¶</a></dt>
+<dd><div class="versionadded">
+<p><span class="versionmodified added">New in version 6.3.0.</span></p>
+</div>
+<p>Skip a step of the pipeline when it is followed in the forward path.</p>
+<p>The following example shows a combined use of <a class="reference internal" href="conversions/push.html#push"><span class="std std-ref">push</span></a> and <a class="reference internal" href="conversions/pop.html#pop"><span class="std std-ref">pop</span></a> operators,
+with <code class="docutils literal notranslate"><span class="pre">omit_fwd</span></code> and <code class="docutils literal notranslate"><span class="pre">omit_inv</span></code> options, to implement a vertical adjustment that must
+be done in a interpolation CRS that is different from the horizontal CRS
+used in input and output. +omit_fwd in the forward path avoid a useless
+inverse horizontal transformation and relies on the pop operator to restore
+initial horizontal coordinates. +omit_inv serves the similar purpose when
+the pipeline is executed in the reverse direction</p>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
++step +proj=unitconvert +xy_in=deg +xy_out=rad
++step +proj=push +v_1 +v_2
++step +proj=hgridshift +grids=nvhpgn.gsb +omit_inv
++step +proj=vgridshift +grids=g1999u05.gtx +multiplier=1
++step +inv +proj=hgridshift +grids=nvhpgn.gsb +omit_fwd
++step +proj=pop +v_1 +v_2
++step +proj=unitconvert +xy_in=rad +xy_out=deg
+</pre></div>
+</div>
+</dd></dl>
+
+<dl class="std option">
+<dt class="sig sig-object std" id="cmdoption-arg-omit_inv">
+<span id="cmdoption-arg-omit-inv"></span><span class="sig-name descname"><span class="pre">+omit_inv</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-omit_inv" title="Permalink to this definition">¶</a></dt>
+<dd><div class="versionadded">
+<p><span class="versionmodified added">New in version 6.3.0.</span></p>
+</div>
+<p>Skip a step of the pipeline when it is followed in the reverse path.</p>
+</dd></dl>
+
+</section>
+</section>
+</section>
+
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author	PROJ deploybot <proj.bot@proj.bot>	2022-03-22 20:00:06 +0000
committer	PROJ deploybot <proj.bot@proj.bot>	2022-03-22 20:00:06 +0000
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