title: Storing custom data in KML ExtendedData
body: <p>I'm writing some code to output a KML file, with certain application-specific info stored in the ExtendedData nodes.  As an easy way to debug the validity of my KML, I'm using Google Earth with the "Show prompts for all errors" option enabled (<a href="http://screencast.com/t/UDkJc6Nv5G4" rel="nofollow">http://screencast.com/t/UDkJc6Nv5G4</a>).</p>
  
  <p>I'm trying to write the ExtendedData as untyped arbitrary tags, as documented in the KML reference under <em>namespace_prefix:other</em> (see: <a href="https://developers.google.com/kml/documentation/kmlreference?csw=1#extendeddata" rel="nofollow">https://developers.google.com/kml/documentation/kmlreference?csw=1#extendeddata</a>)   The problem is, even if I replicate their sample code <em>precisely</em>, Google Earth complains about an unknown element: <a href="http://screencast.com/t/bCMgaDiDYdWt" rel="nofollow">http://screencast.com/t/bCMgaDiDYdWt</a>.</p>
  
  <p>Here's the complete KML:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;kml xmlns="http://www.opengis.net/kml/2.2"&gt;
  &lt;Document&gt;
      &lt;name&gt;Sample KML.kml&lt;/name&gt;
      &lt;Placemark&gt;
          &lt;name&gt;Test KML&lt;/name&gt;
          &lt;Point&gt;
              &lt;coordinates&gt;-118.405022621,33.943893977,0&lt;/coordinates&gt;
          &lt;/Point&gt;
          &lt;ExtendedData xmlns:prefix="camp"&gt;
              &lt;camp:number&gt;14&lt;/camp:number&gt;
              &lt;camp:parkingSpaces&gt;2&lt;/camp:parkingSpaces&gt;
              &lt;camp:tentSites&gt;4&lt;/camp:tentSites&gt;
          &lt;/ExtendedData&gt;
      &lt;/Placemark&gt;
  &lt;/Document&gt;
  &lt;/kml&gt;
  </code></pre>
  
  <p>I'm aware that there are alternative ways to include application-specific data (i.e. &lt;Data&gt;, &lt;SchemaData&gt;, etc) - but for this question, I'm interested in figuring out why Google's own software reports this as invalid, when it comes straight from their own example.  Have I left something else out of the KML, or am I misinterpreting the docs?</p>