Variable Substitution in KML Icon Reference - kml

Question

How can I do variable substitution in a kml icon reference? I'm using Google Earth to load the kml, and my image doesn't appear for this simple example:
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
<Document>
<name>TestMap</name>
<Style id="Icon1">
<IconStyle>
<Icon>
<href>$[url]</href>
</Icon>
</IconStyle>
</Style>
<Placemark>
<name>Hello World</name>
<styleUrl>#Icon1</styleUrl>
<ExtendedData>
<Data name="url">
<value>http://magiccards.info/scans/en/al/232.jpg</value>
</Data>
</ExtendedData>
<Point>
<coordinates>
0,0,0
</coordinates>
</Point>
</Placemark>
</Document>
</Document>
</kml>

Answer 1

Variable substitution for extended data in KML only works in context of the description so you could show the placemark's data url via the description balloon.
<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
<name>Data+BalloonStyle</name>
<Style id="balloon-style">
<BalloonStyle>
<text>
<![CDATA[
$[name]<br>
<img src="$[url]"/>
]]>
</text>
</BalloonStyle>
</Style>
<Placemark>
<name>Hello World</name>
<styleUrl>#balloon-style</styleUrl>
<ExtendedData>
<Data name="url">
<value>http://magiccards.info/scans/en/al/232.jpg</value>
</Data>
</ExtendedData>
<Point>
<coordinates>-111.956,33.5043</coordinates>
</Point>
</Placemark>
</Document>
</kml>
See related tutorial for adding custom data which describes using the BalloonStyle Element as a Template
https://developers.google.com/kml/documentation/extendeddata
If you want to display a custom icon via IconStyle per placemark then you need to define an inline Style for each placemark with the appropriate URL.
<Placemark>
<name>Hello World</name>
<Style>
<IconStyle>
<Icon>
<href>http://magiccards.info/scans/en/al/232.jpg</href>
</Icon>
</IconStyle>
</Style>
<Point>
<coordinates>-111.956,33.5043</coordinates>
</Point>
</Placemark>