Tom Anthony

Notice: As a few people have pointed out, this announcement from Google means Geocoding is now built in. Yet as more people have pointed out – it kinda sucks accuracy wise (think over a mile off on some postcodes!), whereas my method continues to be accurate.

Google Maps API provides a geocoding feature, for finding the latitude and longitude of places or addresses; but it does not work for UK postcodes. This is thanks to Royal Mail who have a copyright on the data, and are very restrictive with their (expensive) licenses for it.

There are various solutions out there for using 3rd party services and importing the data to be used with Google Maps, or for using community built databases for the info. However, I’ve had a few people ask me about doing it just though Google.

It is possible — Google AJAX Search API does provide geocoding for UK postcodes. We need to use the two APIs in harmony to achieve our result.

So here it is.

Step by step

I’ll assume you already know how to use Google Maps API, and you came here just looking how to add geocoding for the UK.

Step 1.

Grab a two API keys, if you already have your Google Maps API key, just grab an AJAX search key. You can get them here:

http://www.google.com/apis/maps/signup.html

http://code.google.com/apis/ajaxsearch/signup.html

Step 2.

Google will give you a sample page, you need to stick your two API keys at the top of the page, followed by a reference to your Javascript file:

<script src="http://maps.google.com/maps?file=api&v=2&key=*KEY*"
type="text/javascript"></script>

<script src="http://www.google.com/uds/api?file=uds.js&v=1.0&key=*KEY*"
type="text/javascript"></script>

<script src="gmap.js" type="text/javascript"></script>

Ensure the reference to your Javascript file comes after the two API keys.

Step 3.

In addition to the Google Maps API stuff, you need to stick a reference to Google local search at the top of your Javascript file:

var localSearch = new GlocalSearch();

You can grab my Javascript file right here, but remember you’ll need to change the API keys.

Step 4.

The key to this Geocoder is only a single function:

function usePointFromPostcode(postcode, callbackFunction) {
  
  localSearch.setSearchCompleteCallback(null, 
    function() {
      
      if (localSearch.results[0]) {    
        var resultLat = localSearch.results[0].lat;
        var resultLng = localSearch.results[0].lng;
        var point = new GLatLng(resultLat,resultLng);
        callbackFunction(point);
      }else{
        alert("Postcode not found!");
      }
    });  
    
  localSearch.execute(postcode + ", UK");
}

It takes 2 arguments; postcode is the postcode you want to look for, and callbackFunction is the function you wish to run on the results.

Why is it necessary to do it this way? It is the way AJAX, and thus Google AJAX Search API, works – the request is sent, and a callback function is designated to handle the results returned, when they are ready.

In our case, the callback function can do whatever you want with the results, which will come in the format of a GLatLng (often just called a point); I’ve supplied 2 sample functions, placeMarkerAtPoint and setCenterToPoint which do pretty much what they sound like they do.

Step 5.

Putting aside accessibility and graceful degradation for the sake of simplicity in this tutorial, the last step we need is just to add some hooks into our Javascript:

<input type="text" id="postcode" size="10" />
<input type="submit" value="Place Marker" onclick="javascript:
usePointFromPostcode(document.getElementById('postcode').value, placeMarkerAtPoint)" />

We have a field for inputting a postcode, and I’ve added a button for placing a marker there. Where I have placeMarkerAtPoint you can put a reference to your own function, or you can even add a function right in there, like this:

  <input type="submit" value="Do whatever" onclick="javascript:
  usePointFromPostcode(document.getElementById('postcode').value,
    function (point) {
      alert('Latitude: ' + point.lat() + '\nLongitude: ' + point.lng());
      })" />

Demo

If you are coming in from an RSS reader, either visit this blog post on the site, or see the demo page.

Postcode:

Conclusion

Until Royal Mail sort get their act together, and relax the licensing agreement, hopefully this will help people who want a ‘pure’ Google solution and hadn’t come across this option. Please use the comments section to let me know if you are using this, or if you have any improvements or suggestions.