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<section id="example-1-6">
<span id="example1-6"></span><h1>Example 1.6<a class="headerlink" href="#example-1-6" title="Link to this heading">¶</a></h1>
<section id="defining-projections-and-extents">
<h2>Defining Projections and Extents<a class="headerlink" href="#defining-projections-and-extents" title="Link to this heading">¶</a></h2>
<img alt="https://demo.mapserver.org/cgi-bin/mapserv?map=/mapserver/apps/tutorial/htdocs/example1-6.map&layer=states&layer=states_label&layer=modis&mode=map" src="https://demo.mapserver.org/cgi-bin/mapserv?map=/mapserver/apps/tutorial/htdocs/example1-6.map&layer=states&layer=states_label&layer=modis&mode=map" />
<p>Whoa! What happened here?</p>
<p>You’ve just experienced on-the-fly reprojection. When a PROJECTION object is
defined in the mapfile, MapServer passes the layer and projection info to the
PROJ library (formerly <cite>PROJ.4</cite>) which does the reprojection. For more information about the
PROJ library, please visit <a class="reference external" href="https://proj.org/">https://proj.org/</a>.</p>
<p>This example attempts to shed some light on the projection support in MapServer.</p>
<p>This is what the mapfile looks like: <a class="reference internal" href="example1-6-map.html#example1-6-map"><span class="std std-ref">Example1-6.map</span></a></p>
<p>The first thing you might have noticed with our mapfile is the original EXTENT
has been commented out and the new EXTENT values don’t look anything like
latitude and longitude values. Also, we have added a PROJECTION object near the
top and within each of our layers.</p>
<p>Let’s have a look at the new object and parameter:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">EXTENT</span> <span class="mf">201621.496941</span> <span class="o">-</span><span class="mf">294488.285333</span> <span class="mf">1425518.020722</span> <span class="mf">498254.511514</span>
</pre></div>
</div>
<p>The extent we provide to MapServer needs to be in the same units as the output
projection. Since Lambert Azimuthal Equal-Area’s units are in meters, we have
to give the new extent in meters. We can calculate the new extent using
QGIS or some other GIS package, or you can use PROJ’s cs2cs utility. The
command below is what can be used to reproject the original extent values:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">cs2cs</span> <span class="o">+</span><span class="n">proj</span><span class="o">=</span><span class="n">latlong</span> <span class="o">+</span><span class="n">datum</span><span class="o">=</span><span class="n">WGS84</span> <span class="o">+</span><span class="n">to</span> <span class="o">+</span><span class="n">proj</span><span class="o">=</span><span class="n">laea</span> <span class="o">+</span><span class="n">ellps</span><span class="o">=</span><span class="n">clrk66</span> <span class="o">+</span><span class="n">lat_0</span><span class="o">=</span><span class="mi">45</span> <span class="o">+</span><span class="n">lon_0</span><span class="o">=-</span><span class="mi">100</span>
</pre></div>
</div>
<p>After typing the command, enter the southwestern coordinate pair (the lower
left coordinates), separated by a space:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="mf">97.5</span> <span class="mf">41.619778</span>
</pre></div>
</div>
<p>to which the „cs2cs“ utility gives the values:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mf">208398.01</span> <span class="o">-</span><span class="mf">372335.44</span> <span class="mf">0.000</span>
</pre></div>
</div>
<p>I then type the northeastern coordinate pair (upper right corner coordinates),
again separated by space:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="mf">82.122902</span> <span class="mf">49.38562</span>
</pre></div>
</div>
<p>and the following values are returned:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mf">1285308.08</span> <span class="mf">632638.93</span> <span class="mf">0.000</span>
</pre></div>
</div>
<p>You will notice that „cs2cs“ returns a set of three values. You can ignore the
third value, 0.000, as it means to represent altitude (which we’re not using).
Anyway, we can now present the EXTENT as:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">EXTENT</span> <span class="mf">208398.01</span> <span class="o">-</span><span class="mf">372335.44</span> <span class="mf">1285308.08</span> <span class="mf">632638.93</span>
</pre></div>
</div>
<p>But it doesn’t match up the extent above, you say. Well, that’s what usually
happens when you reproject – the map doesn’t necessarily get centered as you’d
expect. You can fudge around it, taking a few thousand meters off to the left
and adding a few to the right. Or, you can use a graphical GIS package to give
you the extent. <a class="reference external" href="https://gis.stackexchange.com/questions/85192/getting-extent-of-layer-in-qgis">Here are the instructions</a>
for getting <cite>EXTENT</cite> using QGIS.</p>
<p>MapServer can take projection definitions in two ways. The first is shown in the
mapfile’s output PROJECTION object (the first PROJECTION block). This is the
traditional way of passing parameters to the PROJ library. The other way is
to use the EPSG codes. These codes are standard projection codes (or spatial
reference identifiers) as defined by the European Petroleum Survey Group
(EPSG). In the case of our Lambert Azimuthal Equal-Area projection, it has a
code of „2163“. If you comment out the four lines after PROJECTION and uncomment
the line „init=epsg:2163“, this will provide the same information to PROJ. If
you want to learn more about EPSG codes, have a look at
„/usr/local/share/proj/epsg“ or „C:/proj/nad/epsg“ („/ms4w/proj/nad“ in MS4W).</p>
<div class="admonition note">
<p class="admonition-title">Bemerkung</p>
<p>You can search for EPSG codes and see MapServer usage examples at <a class="reference external" href="https://spatialreference.org/">https://spatialreference.org/</a></p>
</div>
<p>Also, check out the EPSG web site at <a class="reference external" href="https://epsg.org">https://epsg.org</a>.</p>
<p>If you need to learn more about projections, please look through some of the links below:</p>
<ul class="simple">
<li><p><a class="reference external" href="http://www.geography.hunter.cuny.edu/mp/">http://www.geography.hunter.cuny.edu/mp/</a></p></li>
<li><p><a class="reference external" href="https://en.wikipedia.org/wiki/Map_projection">https://en.wikipedia.org/wiki/Map_projection</a></p></li>
<li><p><a class="reference external" href="https://store.usgs.gov/assets/mod/storefiles/PDF/16573.pdf">https://store.usgs.gov/assets/mod/storefiles/PDF/16573.pdf</a></p></li>
</ul>
<p>You can find more by searching the web for „map projection“.</p>
<hr class="docutils" />
<p><a class="reference internal" href="example1-5.html#example1-5"><span class="std std-ref">Back to Example 1.5</span></a> | <a class="reference internal" href="example1-7.html#example1-7"><span class="std std-ref">Proceed to Example 1.7</span></a></p>
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<li><a class="reference internal" href="#">Example 1.6</a><ul>
<li><a class="reference internal" href="#defining-projections-and-extents">Defining Projections and Extents</a></li>
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