A geocoder is a system that takes a street address and converts it to an (approximate) latitude and longitude for use in GPS navigation, online cartography, and similar applications. MapQuest has a nice online tutorial for different geocoding algorithms, with illustrations.

A new, extremely simple geocoder has just come online for Canada, named (unsurprisingly) geocoder.ca. The app includes both an HTML interface and an XML (and HTTP) API. Here is an example of a GET URL for a simple API call to geocode 1 Lindbergh, Ottawa, Ontario:

http://www.geocode.ca/?addresst=lindbergh&stno=1&city=ottawa&prov=ON&geoit=xml

Here’s the XML document returned in the HTTP response:

<?xml version="1.0" encoding="iso-8859-1" ?>
<geodata>
        <latt>45.3358320000</latt>
        <longt>-75.6864510000</longt>
</geodata>

It’s also possible to send your own transaction ID and have it returned in the XML. If I append “&id=foobar” to the above URL, I get this back:

<?xml version="1.0" encoding="iso-8859-1" ?>
<geodata>
        <latt>45.3358320000</latt>
        <longt>-75.6864510000</longt>
	<id>foobar</id>
</geodata>

Finally, like many REST applications, this one has its own ad-hoc error-reporting scheme:

<?xml version="1.0" encoding="iso-8859-1" ?>
<geodata>
        <error>
                <code>004</code>
                <description>Specify a Canadian province two letter code.</description>
        </error>
        <latt>
        <longt>-

</geodata>

This is another great example of how plain XML and HTTP make it easy to provide a public API.