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<section id="geocentric-grid-shift">
<span id="xyzgridshift"></span><h1>Geocentric grid shift<a class="headerlink" href="#geocentric-grid-shift" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 7.0.0.</span></p>
</div>
<p>Geocentric translation using a grid shift</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>xyzgridshift</p></td>
</tr>
<tr class="row-even"><td><p><strong>Domain</strong></p></td>
<td><p>3D</p></td>
</tr>
<tr class="row-odd"><td><p><strong>Input type</strong></p></td>
<td><p>Cartesian coordinates</p></td>
</tr>
<tr class="row-even"><td><p><strong>Output type</strong></p></td>
<td><p>Cartesian coordinates</p></td>
</tr>
</tbody>
</table>
<p>Perform a geocentric translation by bilinear interpolation of dx, dy, dz
translation values from a grid. The grid is referenced against either the
2D geographic CRS corresponding to the input (or sometimes output) CRS.</p>
<p>This method is described (in French) in <span id="id1">[<a class="reference internal" href="../../zreferences.html#id26" title="IGN. Grille de parametres de transformation de coordonnees - GR3DF97A - notice d'utilisation. Technical Report, Service de Geodesie et Nivellement, Institut Geographique National, 1997. URL: https://geodesie.ign.fr/contenu/fichiers/documentation/algorithmes/notice/NTG_88.pdf.">NTF_88</a>]</span>
and as EPSG operation method code 9655 in <span id="id2">[<a class="reference internal" href="../../zreferences.html#id17" title="IOGP. Geomatics guidance note 7, part 2: coordinate conversions & transformations including formulas. IOGP Publication 373-7-2, International Association For Oil And Gas Producers, 2018. URL: https://www.iogp.org/bookstore/product/coordinate-conversions-and-transformation-including-formulas/.">IOGP2018</a>]</span> (§2.4.4.1.1
France geocentric interpolation).</p>
<p>The translation in the grids are added to the input coordinates in the forward direction,
and subtracted in the reverse direction.
By default (if <code class="docutils literal notranslate"><span class="pre">grid_ref=input_crs</span></code>), in the forward direction, the input coordinates
are converted to their geographic equivalent to directly read and interpolate from
the grid. In the reverse direction, an iterative method is used to be able to find
the grid locations to read.
If <code class="docutils literal notranslate"><span class="pre">grid_ref=output_crs</span></code> is used, then the reverse strategy is applied: iterative
method in the forward direction, and direct read in the reverse direction.</p>
<section id="example">
<h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<p>NTF to RGF93 transformation using <code class="file docutils literal notranslate"><span class="pre">gr3df97a.tif</span></code> grid</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+proj=pipeline
+step +proj=push +v_3
+step +proj=cart +ellps=clrk80ign
+step +proj=xyzgridshift +grids=gr3df97a.tif +grid_ref=output_crs +ellps=GRS80
+step +proj=cart +ellps=GRS80 +inv
+step +proj=pop +v_3
</pre></div>
</div>
<section id="parameters">
<h3>Parameters<a class="headerlink" href="#parameters" title="Permalink to this headline">¶</a></h3>
<p>The ellipsoid parameters should be the ones consistent with <code class="docutils literal notranslate"><span class="pre">grid_ref</span></code>.
They are used to perform a geocentric to geographic conversion to find the
translation parameters.</p>
<section id="required">
<h4>Required<a class="headerlink" href="#required" title="Permalink to this headline">¶</a></h4>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-ellps">
<span class="sig-name descname"><span class="pre">+ellps</span></span><span class="sig-prename descclassname"><span class="pre">=<value></span></span><a class="headerlink" href="#cmdoption-arg-ellps" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of a built-in ellipsoid definition.</p>
<p>See <a class="reference internal" href="../../usage/ellipsoids.html#ellipsoids"><span class="std std-ref">Ellipsoids</span></a> for more information, or execute
<a class="reference internal" href="../../apps/proj.html#cmdoption-proj-le"><code class="xref std std-option docutils literal notranslate"><span class="pre">proj</span> <span class="pre">-le</span></code></a> for a list of built-in ellipsoid names.</p>
<p><em>Defaults to “GRS80”.</em></p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-grids">
<span class="sig-name descname"><span class="pre">+grids</span></span><span class="sig-prename descclassname"><span class="pre">=<list></span></span><a class="headerlink" href="#cmdoption-arg-grids" title="Permalink to this definition">¶</a></dt>
<dd><p>Comma-separated list of grids to load. If a grid is prefixed by an <code class="docutils literal notranslate"><span class="pre">@</span></code> the
grid is considered optional and PROJ will the not complain if the grid is
not available.</p>
<p>Grids are expected to be in GeoTIFF format. If no metadata is provided,
the first, second and third samples are assumed to be the geocentric
translation along X, Y and Z axis respectively, in metres.</p>
</dd></dl>
</section>
<section id="optional">
<h4>Optional<a class="headerlink" href="#optional" title="Permalink to this headline">¶</a></h4>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-grid_ref">
<span id="cmdoption-arg-grid-ref"></span><span class="sig-name descname"><span class="pre">+grid_ref</span></span><span class="sig-prename descclassname"><span class="pre">=input_crs/output_crs</span></span><a class="headerlink" href="#cmdoption-arg-grid_ref" title="Permalink to this definition">¶</a></dt>
<dd><p>Specify in which CRS the grid is referenced to. The default value is
input_crs. That is the grid is referenced in the geographic CRS corresponding
to the input geocentric CRS.</p>
<p>If output_crs is specified, the grid is referenced in the geographic CRS corresponding
to the output geocentric CRS. This is for example the case for the French
<code class="file docutils literal notranslate"><span class="pre">gr3df97a.tif</span></code> grid converting from NTF to RGF93, but referenced against RGF93.
Thus in the forward direction (NTF->RGF93), an iterative method is used to find
the appropriate shift.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-arg-multiplier">
<span class="sig-name descname"><span class="pre">+multiplier</span></span><span class="sig-prename descclassname"><span class="pre">=<value></span></span><a class="headerlink" href="#cmdoption-arg-multiplier" title="Permalink to this definition">¶</a></dt>
<dd><p>Specify the multiplier to apply to the grid values. Defaults to 1.0</p>
</dd></dl>
</section>
</section>
</section>
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