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<div class="section" id="plotting-sandwell-smith-mercator-img-grids">
<span id="example-27"></span><h1>(27) Plotting Sandwell/Smith Mercator img grids<a class="headerlink" href="#plotting-sandwell-smith-mercator-img-grids" title="Permalink to this headline">¶</a></h1>
<p>Next, we show how to plot a data grid that is distributed in projected
form. The gravity and predicted bathymetry grids produced by David
Sandwell and Walter H. F. Smith are not geographical grids but instead
given on a spherical Mercator grid. The GMT supplement imgsrc has
tools to extract subsets of these large grids. If you need to make a
non-Mercator map then you must extract a geographic grid using
<a class="reference internal" href="../supplements/img/img2grd.html"><span class="doc">img2grd</span></a> and then plot it using your
desired map projection. However, if you want to make a Mercator map then
you can save time and preserve data quality by avoiding to re-project
the data set twice since it is already in a Mercator projection. This
example shows how this is accomplished. We use the <strong>-M</strong> option in
<a class="reference internal" href="../supplements/img/img2grd.html"><span class="doc">img2grd</span></a> <a class="footnote-reference" href="#id2" id="id1">[1]</a> to pull out the grid
in Mercator units (i.e., do <em>not</em> invert the Mercator projection) and
then simply plot the grid using a linear projection with a suitable
scale (here 0.25 inches per degrees of longitude). To overlay basemaps
and features that has geographic longitude/latitude coordinates we must
remember two key issues:</p>
<ol class="arabic simple">
<li>This is a <em>spherical</em> Mercator grid so we must use
--<strong>PROJ_ELLIPSOID</strong>=Sphere with all commands that involve
projections (or use <a class="reference internal" href="../gmtset.html"><span class="doc">gmtset</span></a> to change the setting).</li>
<li>Select Mercator projection and use the same scale that was used with
the linear projection.</li>
</ol>
<p>This map of the Tasman Sea shows the marine gravity anomalies with land
painted black. A color scale bar was then added to complete the illustration.</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/bin/bash</span>
<span class="c1"># GMT EXAMPLE 27</span>
<span class="c1">#</span>
<span class="c1"># Purpose: Illustrates how to plot Mercator img grids</span>
<span class="c1"># GMT modules: makecpt, grdimage, grdinfo, pscoast</span>
<span class="c1"># GMT supplement: img2grd (to read Sandwell/Smith img files)</span>
<span class="c1"># Unix progs: rm</span>
<span class="c1">#</span>
<span class="nv">ps</span><span class="o">=</span>example_27.ps
<span class="c1"># Gravity in tasman_grav.nc is in 0.1 mGal increments and the grid</span>
<span class="c1"># is already in projected Mercator x/y units.</span>
<span class="c1"># Make a suitable cpt file for mGal</span>
gmt makecpt -T-120/120 -Crainbow > grav.cpt
<span class="c1"># Since this is a Mercator grid we use a linear projection</span>
gmt grdimage tasman_grav.nc<span class="o">=</span>ns+s0.1 -I+a45+nt1 -Jx0.25i -Cgrav.cpt -P -K > <span class="nv">$ps</span>
<span class="c1"># Then use gmt pscoast to plot land; get original -R from grid img remark</span>
<span class="c1"># and use Mercator gmt projection with same scale as above on a spherical Earth</span>
<span class="nv">R</span><span class="o">=</span><span class="sb">`</span>gmt grdinfo tasman_grav.nc -Ii<span class="sb">`</span>
gmt pscoast <span class="nv">$R</span> -Jm0.25i -Ba10f5 -BWSne -O -K -Gblack --PROJ_ELLIPSOID<span class="o">=</span>Sphere <span class="se">\</span>
-Cwhite -Dh+ --FORMAT_GEO_MAP<span class="o">=</span>dddF >> <span class="nv">$ps</span>
<span class="c1"># Put a color legend in top-left corner of the land mask</span>
gmt psscale -DjTL+o1c+w2i/0.15i -R -J -Cgrav.cpt -Bx50f10 -By+lmGal -I -O -F+gwhite+p1p >> <span class="nv">$ps</span>
<span class="c1"># Clean up</span>
rm -f grav.cpt
</pre></div>
</div>
<div class="figure align-center" id="id3">
<a class="reference internal image-reference" href="../_images/example_27.png"><img alt="../_images/example_27.png" src="../_images/example_27.png" style="width: 500px;" /></a>
<p class="caption"><span class="caption-text">Plotting Sandwell/Smith Mercator img grids.</span></p>
</div>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>You could also use <a class="reference internal" href="../supplements/img/img2grd.html"><span class="doc">img2grd</span></a>
directly - your only option under DOS</td></tr>
</tbody>
</table>
</div>
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