An Open Postcode Geocoding API

Because I needed one for various (non-profit) projects of my own, I’ve put together a small Postcode geocoding web service which collates partial data from various sources into what aims to be a full geocoder / reverse geocoder for UK postcodes.

Now, the term ‘geocoding’ can mean several very subtly different things depending on who you talk to. So to clarify, this is what the web service does.

1. It takes a full UK postcode
2. It returns a geographical location (lat/lng and optionally os/landranger grid references)
3. It also attempts to reverse geocode the street address via the Google Maps API (experimental)

Isn’t that Illegal?

Not anymore. Ordnance Survey have open sourced huge chunks of their data. This includes their CodePoint file which contains the coordinates of every single UK postcode.

Usage

There’s nothing complicated involved in calling the service. Simply make an HTTP request in the following format, where SW1A 2AA is your postcode; or more accurately that of 10 Downing Street.

Get the data in XML format
http://geo.jamiethompson.co.uk/SW1A2AA.xml

<result>
  <status>200</status>
  <postcode>SW1A 2AA</postcode>
  <geo>
    <os_x>530047</os_x>
    <os_y>179951</os_y>
    <lat>51.50354</lat>
    <lng>-0.127696</lng>
    <landranger>TQ300799</landranger>
    <accuracy>1</accuracy>
  </geo>
  <address>
    <street>Downing St</street>
    <locality>London</locality>
    <district>Westminster</district>
    <county>Greater London</county>
  </address>
</result>

The data is also available as JSON
http://geo.jamiethompson.co.uk/SW1A2AA.json

{
   "status":200,
   "postcode":"SW1A 2AA",
   "geo":{
      "os_x":"530047",
      "os_y":"179951",
      "lat":"51.50354",
      "lng":"-0.127696",
      "landranger":"TQ300799",
      "accuracy":"1"
   },
   "address":{
      "street":"Downing St",
      "locality":"London",
      "district":"Westminster",
      "county":"Greater London"
   }
}

and additionally as JSONP by specifying a callback
http://geo.jamiethompson.co.uk/SW1A2AA.json?callback=doStuff

Things You Should be Aware of

The data returned should be pretty self documenting but there’s a few things you should be aware of.

Although, more often than not you will receive a full dataset. The only guaranteed values are lat and lng. All other values of geo are optional. Additionally to this, if the accuracy level of a result is 2 then the coordinates relate to the broader postcode area rather than the postcode itself. An accuracy 2 result for the postcode JE3 8FA* will return the centre point of the JE3 8 area.

* All channel island postcodes are returned at accuracy level 2

Northern Ireland (BTXX XXX) postcodes don’t return ordnance survey osgrid coords. Lat/lng values however are accurate to level 1.

A final word

This is an experimental web service. Please don’t hammer it for anything humongous or commercial. The reverse geocoding is experimental and cannot be relied upon for anything super-serious at the moment. If you need this data commercially please do yourself (and me) a favour and check out the reasonably priced services offered by the likes of AFD (which I can personally recommend) or PostcodeAnywhere