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authorPROJ deploybot <proj.bot@proj.bot>2022-03-22 20:00:06 +0000
committerPROJ deploybot <proj.bot@proj.bot>2022-03-22 20:00:06 +0000
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tree27e4198db6011e3097eb7bcfe7197684aba7583a /development/quickstart.html
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+
+ <section id="quick-start">
+<span id="dev-quickstart"></span><h1>Quick start<a class="headerlink" href="#quick-start" title="Permalink to this headline">¶</a></h1>
+<p>This is a short introduction to the PROJ API. In the following section we
+create a simple program that transforms a geodetic coordinate to UTM and back
+again. The program is explained a few lines at a time. The complete program can
+be seen at the end of the section.</p>
+<p>See the following sections for more in-depth descriptions of different parts of
+the PROJ API or consult the <a class="reference internal" href="reference/index.html"><span class="doc">API reference</span></a> for specifics.</p>
+<p>Before the PROJ API can be used it is necessary to include the <code class="file docutils literal notranslate"><span class="pre">proj.h</span></code> header
+file. Here <code class="file docutils literal notranslate"><span class="pre">stdio.h</span></code> is also included so we can print some text to the screen:</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;stdio.h&gt;</span><span class="cp"></span>
+<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;proj.h&gt;</span><span class="cp"></span>
+</pre></div>
+</div>
+<p>Let’s declare a few variables that’ll be used later in the program. Each variable
+will be discussed below.
+See the <a class="reference internal" href="reference/datatypes.html"><span class="doc">reference for more info on data types</span></a>.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PJ_CONTEXT</span><span class="w"> </span><span class="o">*</span><span class="n">C</span><span class="p">;</span><span class="w"></span>
+<span class="n">PJ</span><span class="w"> </span><span class="o">*</span><span class="n">P</span><span class="p">;</span><span class="w"></span>
+<span class="n">PJ</span><span class="w"> </span><span class="o">*</span><span class="n">norm</span><span class="p">;</span><span class="w"></span>
+<span class="n">PJ_COORD</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">;</span><span class="w"></span>
+</pre></div>
+</div>
+<p>For use in multi-threaded programs the <a class="reference internal" href="reference/datatypes.html#c.PJ_CONTEXT" title="PJ_CONTEXT"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_CONTEXT</span></code></a> threading-context
+is used. In this particular example it is not needed, but for the sake of
+completeness we demonstrate its use here.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_context_create</span><span class="p">();</span><span class="w"></span>
+</pre></div>
+</div>
+<p>Next we create the <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> transformation object <code class="docutils literal notranslate"><span class="pre">P</span></code> with the function
+<a class="reference internal" href="reference/functions.html#c.proj_create_crs_to_crs" title="proj_create_crs_to_crs"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_create_crs_to_crs()</span></code></a>.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">P</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_create_crs_to_crs</span><span class="w"> </span><span class="p">(</span><span class="n">C</span><span class="p">,</span><span class="w"></span>
+<span class="w"> </span><span class="s">&quot;EPSG:4326&quot;</span><span class="p">,</span><span class="w"></span>
+<span class="w"> </span><span class="s">&quot;+proj=utm +zone=32 +datum=WGS84&quot;</span><span class="p">,</span><span class="w"> </span><span class="cm">/* or EPSG:32632 */</span><span class="w"></span>
+<span class="w"> </span><span class="nb">NULL</span><span class="p">);</span><span class="w"></span>
+
+<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="mi">0</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">P</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
+<span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;Failed to create transformation object.</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"></span>
+<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
+<span class="p">}</span><span class="w"></span>
+</pre></div>
+</div>
+<p>Here we have set up a transformation from geographic coordinates to UTM zone
+32N.</p>
+<p><a class="reference internal" href="reference/functions.html#c.proj_create_crs_to_crs" title="proj_create_crs_to_crs"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_create_crs_to_crs()</span></code></a> takes as its arguments:</p>
+<ul class="simple">
+<li><p>the threading context <code class="docutils literal notranslate"><span class="pre">C</span></code> created above,</p></li>
+<li><p>a string that describes the source coordinate reference system (CRS),</p></li>
+<li><p>a string that describes the target CRS and</p></li>
+<li><p>an optional description of the area of use.</p></li>
+</ul>
+<p>It is recommended to create one threading context per thread used by the
+program. This ensures that all <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> objects created in the same
+context will be sharing resources such as error-numbers and loaded grids.</p>
+<p>If you are sure that <code class="docutils literal notranslate"><span class="pre">P</span></code> will only be used by a single program thread, you
+may pass <code class="docutils literal notranslate"><span class="pre">NULL</span></code> for the threading context. This will assign the default
+thread context to <code class="docutils literal notranslate"><span class="pre">P</span></code>.</p>
+<p>The strings for the source and target CRS may be any of:</p>
+<ul class="simple">
+<li><p>PROJ strings, e.g. <code class="docutils literal notranslate"><span class="pre">+proj=longlat</span> <span class="pre">+datum=WGS84</span> <span class="pre">+type=crs</span></code>,</p></li>
+<li><p>CRS identified by their code, e.g. <code class="docutils literal notranslate"><span class="pre">EPSG:4326</span></code> or
+<code class="docutils literal notranslate"><span class="pre">urn:ogc:def:crs:EPSG::4326</span></code>, or</p></li>
+<li><p>a well-known text (WKT) string, e.g.:</p></li>
+</ul>
+<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>GEOGCRS[&quot;WGS 84&quot;,
+ DATUM[&quot;World Geodetic System 1984&quot;,
+ ELLIPSOID[&quot;WGS 84&quot;,6378137,298.257223563,
+ LENGTHUNIT[&quot;metre&quot;,1]]],
+ PRIMEM[&quot;Greenwich&quot;,0,
+ ANGLEUNIT[&quot;degree&quot;,0.0174532925199433]],
+ CS[ellipsoidal,2],
+ AXIS[&quot;geodetic latitude (Lat)&quot;,north,
+ ORDER[1],
+ ANGLEUNIT[&quot;degree&quot;,0.0174532925199433]],
+ AXIS[&quot;geodetic longitude (Lon)&quot;,east,
+ ORDER[2],
+ ANGLEUNIT[&quot;degree&quot;,0.0174532925199433]],
+ USAGE[
+ SCOPE[&quot;unknown&quot;],
+ AREA[&quot;World&quot;],
+ BBOX[-90,-180,90,180]],
+ ID[&quot;EPSG&quot;,4326]]
+</pre></div>
+</div>
+<div class="admonition warning">
+<p class="admonition-title">Warning</p>
+<p>The use of PROJ strings to describe a CRS is not recommended. One of the
+main weaknesses of PROJ strings is their inability to describe a geodetic
+datum, other than the few ones hardcoded in the <code class="docutils literal notranslate"><span class="pre">+datum</span></code> parameter.</p>
+</div>
+<p><a class="reference internal" href="reference/functions.html#c.proj_create_crs_to_crs" title="proj_create_crs_to_crs"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_create_crs_to_crs()</span></code></a> will return a pointer to a <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a>
+object, or a null pointer in the case of an error. The details of the error
+can be retrieved using <a class="reference internal" href="reference/functions.html#c.proj_context_errno" title="proj_context_errno"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_context_errno()</span></code></a>. See <a class="reference internal" href="errorhandling.html"><span class="doc">Error handling</span></a>
+for further details.</p>
+<p>Now that we have a normalized transformation object in <code class="docutils literal notranslate"><span class="pre">P</span></code>, we can use it
+with <a class="reference internal" href="reference/functions.html#c.proj_trans" title="proj_trans"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_trans()</span></code></a> to transform coordinates from the source CRS to the
+target CRS, but first we will discuss the interpretation of coordinates.</p>
+<p>By default, a <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> transformation object accepts coordinates expressed
+in the units and axis order of the source CRS, and returns transformed
+coordinates in the units and axis order of the target CRS.</p>
+<p>For most geographic CRS, the units will be in degrees. In rare cases, such as
+EPSG:4807 / NTF (Paris), this can be grads. For geographic CRS defined by the
+EPSG authority, the order of coordinates is latitude first, longitude second.
+When using a PROJ string, the order is the reverse; longitude first, latitude
+second.</p>
+<p>For projected CRS, the units may vary (metre, us-foot, etc.). For projected CRS
+defined by the EPSG authority, and with EAST / NORTH directions, the order
+might be easting first, northing second, or the reverse. When using a PROJ
+string, the order will be easting first, northing second, except if the
+<code class="docutils literal notranslate"><span class="pre">+axis</span></code> parameter modifies it.</p>
+<p>If you prefer to work with a uniform axis order, regardless of the axis orders
+mandated by the source and target CRS, you can use the
+<a class="reference internal" href="reference/functions.html#c.proj_normalize_for_visualization" title="proj_normalize_for_visualization"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_normalize_for_visualization()</span></code></a> function.</p>
+<p><a class="reference internal" href="reference/functions.html#c.proj_normalize_for_visualization" title="proj_normalize_for_visualization"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_normalize_for_visualization()</span></code></a> takes a threading context and an
+existing <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> object, and generates from it a new <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> that
+accepts as input and returns as output coordinates using the traditional GIS
+order. That is, longitude followed by latitude, optionally followed by
+elevation and time for geographic CRS, and easting followed by northing for
+most projected CRS.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">norm</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_normalize_for_visualization</span><span class="p">(</span><span class="n">C</span><span class="p">,</span><span class="w"> </span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="mi">0</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">norm</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
+<span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;Failed to normalize transformation object.</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"></span>
+<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
+<span class="p">}</span><span class="w"></span>
+<span class="n">proj_destroy</span><span class="p">(</span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="n">P</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">norm</span><span class="p">;</span><span class="w"></span>
+</pre></div>
+</div>
+<p>Next we create a <a class="reference internal" href="reference/datatypes.html#c.PJ_COORD" title="PJ_COORD"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_COORD</span></code></a> coordinate object, using the function
+<a class="reference internal" href="reference/functions.html#c.proj_coord" title="proj_coord"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_coord()</span></code></a>.</p>
+<p>The following example creates a coordinate for 55°N 12°E (Copenhagen).</p>
+<p>Because we have normalized the transformation object with
+<a class="reference internal" href="reference/functions.html#c.proj_normalize_for_visualization" title="proj_normalize_for_visualization"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_normalize_for_visualization()</span></code></a>, the order of coordinates is
+longitude followed by latitude, and the units are degrees.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_coord</span><span class="p">(</span><span class="mi">12</span><span class="p">,</span><span class="w"> </span><span class="mi">55</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span><span class="w"></span>
+</pre></div>
+</div>
+<p>Now we are ready to transform the coordinate into UTM zone 32, using the
+function <a class="reference internal" href="reference/functions.html#c.proj_trans" title="proj_trans"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_trans()</span></code></a>.</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_trans</span><span class="p">(</span><span class="n">P</span><span class="p">,</span><span class="w"> </span><span class="n">PJ_FWD</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">);</span><span class="w"></span>
+<span class="n">printf</span><span class="p">(</span><span class="s">&quot;easting: %.3f, northing: %.3f</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">enu</span><span class="p">.</span><span class="n">e</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">enu</span><span class="p">.</span><span class="n">n</span><span class="p">);</span><span class="w"></span>
+</pre></div>
+</div>
+<p><a class="reference internal" href="reference/functions.html#c.proj_trans" title="proj_trans"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_trans()</span></code></a> takes as its arguments:</p>
+<ul class="simple">
+<li><p>a <a class="reference internal" href="reference/datatypes.html#c.PJ" title="PJ"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ</span></code></a> transformation object,</p></li>
+<li><p>a <a class="reference internal" href="reference/datatypes.html#c.PJ_DIRECTION" title="PJ_DIRECTION"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_DIRECTION</span></code></a> direction, and</p></li>
+<li><p>the <a class="reference internal" href="reference/datatypes.html#c.PJ_COORD" title="PJ_COORD"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_COORD</span></code></a> coordinate to transform.</p></li>
+</ul>
+<p>The direction argument can be one of:</p>
+<ul class="simple">
+<li><p><code class="docutils literal notranslate"><span class="pre">PJ_FWD</span></code> – “forward” transformation from source CRS to target CRS.</p></li>
+<li><p><code class="docutils literal notranslate"><span class="pre">PJ_IDENT</span></code> – “identity”, return the source coordinate unchanged.</p></li>
+<li><p><code class="docutils literal notranslate"><span class="pre">PJ_INV</span></code> – “inverse” transformation from target CRS to source CRS.</p></li>
+</ul>
+<p>It returns the new transformed <a class="reference internal" href="reference/datatypes.html#c.PJ_COORD" title="PJ_COORD"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_COORD</span></code></a> coordinate.</p>
+<p>We can perform the transformation in reverse (from UTM zone 32 back to
+geographic) as follows:</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_trans</span><span class="p">(</span><span class="n">P</span><span class="p">,</span><span class="w"> </span><span class="n">PJ_INV</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span><span class="w"></span>
+<span class="n">printf</span><span class="p">(</span><span class="s">&quot;longitude: %g, latitude: %g</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">lp</span><span class="p">.</span><span class="n">lam</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">lp</span><span class="p">.</span><span class="n">phi</span><span class="p">);</span><span class="w"></span>
+</pre></div>
+</div>
+<p>Before ending the program, we need to release the memory allocated to our
+objects:</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">proj_destroy</span><span class="p">(</span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="n">proj_context_destroy</span><span class="p">(</span><span class="n">C</span><span class="p">);</span><span class="w"> </span><span class="cm">/* may be omitted in the single threaded case */</span><span class="w"></span>
+</pre></div>
+</div>
+<p>A complete compilable version of the example code can be seen below:</p>
+<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;stdio.h&gt;</span><span class="cp"></span>
+<span class="linenos"> 2</span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;proj.h&gt;</span><span class="cp"></span>
+<span class="linenos"> 3</span>
+<span class="linenos"> 4</span><span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
+<span class="linenos"> 5</span><span class="w"> </span><span class="n">PJ_CONTEXT</span><span class="w"> </span><span class="o">*</span><span class="n">C</span><span class="p">;</span><span class="w"></span>
+<span class="linenos"> 6</span><span class="w"> </span><span class="n">PJ</span><span class="w"> </span><span class="o">*</span><span class="n">P</span><span class="p">;</span><span class="w"></span>
+<span class="linenos"> 7</span><span class="w"> </span><span class="n">PJ</span><span class="w"> </span><span class="o">*</span><span class="n">norm</span><span class="p">;</span><span class="w"></span>
+<span class="linenos"> 8</span><span class="w"> </span><span class="n">PJ_COORD</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">;</span><span class="w"></span>
+<span class="linenos"> 9</span>
+<span class="linenos">10</span><span class="w"> </span><span class="cm">/* or you may set C=PJ_DEFAULT_CTX if you are sure you will */</span><span class="w"></span>
+<span class="linenos">11</span><span class="w"> </span><span class="cm">/* use PJ objects from only one thread */</span><span class="w"></span>
+<span class="linenos">12</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_context_create</span><span class="p">();</span><span class="w"></span>
+<span class="linenos">13</span>
+<span class="linenos">14</span><span class="w"> </span><span class="n">P</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_create_crs_to_crs</span><span class="w"> </span><span class="p">(</span><span class="n">C</span><span class="p">,</span><span class="w"></span>
+<span class="linenos">15</span><span class="w"> </span><span class="s">&quot;EPSG:4326&quot;</span><span class="p">,</span><span class="w"></span>
+<span class="linenos">16</span><span class="w"> </span><span class="s">&quot;+proj=utm +zone=32 +datum=WGS84&quot;</span><span class="p">,</span><span class="w"> </span><span class="cm">/* or EPSG:32632 */</span><span class="w"></span>
+<span class="linenos">17</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">18</span>
+<span class="linenos">19</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="mi">0</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">P</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
+<span class="linenos">20</span><span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;Failed to create transformation object.</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">21</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
+<span class="linenos">22</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
+<span class="linenos">23</span>
+<span class="linenos">24</span><span class="w"> </span><span class="cm">/* This will ensure that the order of coordinates for the input CRS */</span><span class="w"></span>
+<span class="linenos">25</span><span class="w"> </span><span class="cm">/* will be longitude, latitude, whereas EPSG:4326 mandates latitude, */</span><span class="w"></span>
+<span class="linenos">26</span><span class="w"> </span><span class="cm">/* longitude */</span><span class="w"></span>
+<span class="linenos">27</span><span class="w"> </span><span class="n">norm</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_normalize_for_visualization</span><span class="p">(</span><span class="n">C</span><span class="p">,</span><span class="w"> </span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">28</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="mi">0</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">norm</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
+<span class="linenos">29</span><span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;Failed to normalize transformation object.</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">30</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
+<span class="linenos">31</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
+<span class="linenos">32</span><span class="w"> </span><span class="n">proj_destroy</span><span class="p">(</span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">33</span><span class="w"> </span><span class="n">P</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">norm</span><span class="p">;</span><span class="w"></span>
+<span class="linenos">34</span>
+<span class="linenos">35</span><span class="w"> </span><span class="cm">/* a coordinate union representing Copenhagen: 55d N, 12d E */</span><span class="w"></span>
+<span class="linenos">36</span><span class="w"> </span><span class="cm">/* Given that we have used proj_normalize_for_visualization(), the order of</span>
+<span class="linenos">37</span><span class="cm"> /* coordinates is longitude, latitude, and values are expressed in degrees. */</span><span class="w"></span>
+<span class="linenos">38</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_coord</span><span class="p">(</span><span class="mi">12</span><span class="p">,</span><span class="w"> </span><span class="mi">55</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">39</span>
+<span class="linenos">40</span><span class="w"> </span><span class="cm">/* transform to UTM zone 32, then back to geographical */</span><span class="w"></span>
+<span class="linenos">41</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_trans</span><span class="p">(</span><span class="n">P</span><span class="p">,</span><span class="w"> </span><span class="n">PJ_FWD</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">42</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;easting: %.3f, northing: %.3f</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">enu</span><span class="p">.</span><span class="n">e</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">enu</span><span class="p">.</span><span class="n">n</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">43</span>
+<span class="linenos">44</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_trans</span><span class="p">(</span><span class="n">P</span><span class="p">,</span><span class="w"> </span><span class="n">PJ_INV</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">45</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;longitude: %g, latitude: %g</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">lp</span><span class="p">.</span><span class="n">lam</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">lp</span><span class="p">.</span><span class="n">phi</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">46</span>
+<span class="linenos">47</span><span class="w"> </span><span class="cm">/* Clean up */</span><span class="w"></span>
+<span class="linenos">48</span><span class="w"> </span><span class="n">proj_destroy</span><span class="p">(</span><span class="n">P</span><span class="p">);</span><span class="w"></span>
+<span class="linenos">49</span><span class="w"> </span><span class="n">proj_context_destroy</span><span class="p">(</span><span class="n">C</span><span class="p">);</span><span class="w"> </span><span class="cm">/* may be omitted in the single threaded case */</span><span class="w"></span>
+<span class="linenos">50</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
+<span class="linenos">51</span><span class="p">}</span><span class="w"></span>
+</pre></div>
+</div>
+</section>
+
+
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