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<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>
</div>
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