5.3 Geocoding your data

Geocoding refers to the way in which the actual location of a data point (in terms of latitude and longitude co-ordinates, map grid references, or some other reference scheme that allows the data point to be plotted on a map) is obtained from the name of the location, its address, or its postcode. In turn, reverse geocoding refers to the process of taking a map location or co-ordinate and identifying the corresponding address, postcode or ‘toponym’ (that is, the place name).

There is a wide variety of geocoding web services available that can accept either a single address or a set of addresses and return an appropriately geocoded result.

Online map-based search tools all perform some sort of geocoding of addresses or postcodes in order to display locations on the map. For example, you could try typing an address you know into the search box on Google Maps or Yahoo! Maps – does it locate the address properly?

Although it is quite easy to find geocoding APIs for addresses in the USA, thus allowing the creation of applications that can automatically geocode everyday addresses, in the UK the Ordnance Survey and the Post Office have traditionally published UK geolocation data under commercial terms. However, with the move to open up public data it is now possible to access a range of geolocation services in the UK as Linked Data [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

Web developers typically access geocoding APIs in order to geocode locations in a programmatic way. The Yahoo! Placemaker™ API provides a location-extracting and geocoding web service that can be accessed via a URL. Pass in an address, or a block of text containing a placename, and it will identify the address and return latitude and longitude data for it. Many social networks make use of geocoding services to allow users to search for people near a particular location.