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<section id="pop-coordinate-value-to-pipeline-stack">
<span id="pop"></span><h1>Pop coordinate value to pipeline stack<a class="headerlink" href="#pop-coordinate-value-to-pipeline-stack" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 6.0.0.</span></p>
</div>
<p>Retrieve components of a coordinate that was saved in a previous pipeline step.</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 27%" />
<col style="width: 73%" />
</colgroup>
<tbody>
<tr class="row-odd"><td><p><strong>Alias</strong></p></td>
<td><p>pop</p></td>
</tr>
<tr class="row-even"><td><p><strong>Domain</strong></p></td>
<td><p>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>
<p>This operations makes it possible to retrieve coordinate components that was
saved in previous pipeline steps. A retrieved coordinate component is loaded,
or <em>popped</em>, from a memory stack that is part of a <a class="reference internal" href="../pipeline.html#pipeline"><span class="std std-ref">pipeline</span></a>.
The pipeline coordinate stack is inspired by the stack data structure that is
commonly used in computer science. There’s four stacks available: One four each
coordinate dimension. The dimensions, or coordinate components, are numbered
1–4. It is only possible to move data to and from the stack within the same
coordinate component number. Values can be saved to the stack by using the
<a class="reference internal" href="push.html#push"><span class="std std-ref">push operation</span></a>.</p>
<p>If the pop operation is used by itself, e.g. not in a pipeline, it will
function as a no-operation that passes the coordinate through unchanged.
Similarly, if no coordinate component is available on the stack to be popped
the operation does nothing.</p>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<p>A common use of the <a class="reference internal" href="push.html#push"><span class="std std-ref">push</span></a> and pop operations is in 3D
<a class="reference internal" href="../transformations/helmert.html#helmert"><span class="std std-ref">Helmert</span></a> transformations where only the horizontal components
are needed. This is often the case when combining heights from a legacy
vertical reference with a modern geocentric reference. Below is a an example of
such a transformation, where the horizontal part is transformed with a Helmert
operation but the vertical part is kept exactly as the input was.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ echo 12 56 12.3 2020 | cct +proj=pipeline \
+step +proj=push +v_3 \
+step +proj=cart +ellps=GRS80 \
+step +proj=helmert +x=3000 +y=1000 +z=2000 \
+step +proj=cart +ellps=GRS80 +inv \
+step +proj=pop +v_3 \
12.0056753463 55.9866540552 12.3000 2000.0000
</pre></div>
</div>
<p>Note that the third coordinate component in the output is the same as the input.</p>
<p>The same transformation without the push and pop operations would look like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ echo 12 56 12.3 2020 | cct +proj=pipeline \
+step +proj=cart +ellps=GRS80 \
+step +proj=helmert +x=3000 +y=1000 +z=2000 \
+step +proj=cart +ellps=GRS80 +inv \
12.0057 55.9867 3427.7404 2000.0000
</pre></div>
</div>
<p>Here the vertical component is adjusted significantly.</p>
</section>
<section id="parameters">
<h2>Parameters<a class="headerlink" href="#parameters" title="Permalink to this headline">¶</a></h2>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-v_1">
<span id="cmdoption-arg-v-1"></span><span class="sig-name descname"><span class="pre">+v_1</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-v_1" title="Permalink to this definition">¶</a></dt>
<dd><p>Retrieves the first coordinate component from the pipeline stack</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-v_2">
<span id="cmdoption-arg-v-2"></span><span class="sig-name descname"><span class="pre">+v_2</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-v_2" title="Permalink to this definition">¶</a></dt>
<dd><p>Retrieves the second coordinate component from the pipeline stack</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-v_3">
<span id="cmdoption-arg-v-3"></span><span class="sig-name descname"><span class="pre">+v_3</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-v_3" title="Permalink to this definition">¶</a></dt>
<dd><p>Retrieves the third coordinate component from the pipeline stack</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-v_4">
<span id="cmdoption-arg-v-4"></span><span class="sig-name descname"><span class="pre">+v_4</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-arg-v_4" title="Permalink to this definition">¶</a></dt>
<dd><p>Retrieves the fourth coordinate component from the pipeline stack</p>
</dd></dl>
</section>
<section id="further-reading">
<h2>Further reading<a class="headerlink" href="#further-reading" title="Permalink to this headline">¶</a></h2>
<ol class="arabic simple">
<li><p><a class="reference external" href="https://en.wikipedia.org/wiki/Stack_(abstract_data_type)">Stack data structure on Wikipedia</a></p></li>
</ol>
</section>
</section>
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