Spaced Out ::

I’ve been playing around with postcodes a little more recently, particularly since the OS released CodePoint under the open licensing model. I don’t often have recourse to using postcodes given most of my work is with imagery or DEMs, but wanted to do some geocoding. Postcodes aren’t as straightforward as you might at first think and they have a history behind them as well. EDINA has a good post on some of the postcode features.

Address Point provides coordinates for every single delivery address in the UK and these are aggregated in to post codes by the Royal Mail. Address Point is then used to define centroid for each individual postcode. CodePoint Open gives postcode unit centroid which is good for geocoding, but given postcodes notionally represent areas it then raises the question about postcode boundaries. However whilst postcodes are area based, they represent points (i.e. addresses), so how do you define a boundary? From EDINA:

Postcode unit boundaries are a type of synthetic boundary. The postcode unit is a collection of delivery points with the same postcode, while the ‘boundary’ can be drawn anywhere , so long as it contains all delivery points. The postcode unit boundary does not exist in the real world. Where the boundary lines fall makes no difference to the postman, only the delivery points matter. In other words, there is no such thing as a correct postcode boundary.

The Code-Point unit boundaries however, have been created to best represent the postcode unit footprint in a way that allows the dataset to be used for many different applications. ADDRESS-POINT delivery points with sufficient positional quality were used to create the postcode unit polygons, which have been edited to follow major physical features but still enclose each delivery point in the correct postcode unit. Some postcode units do not contain enough delivery points with sufficient positional quality to create an acceptable polygon ‘footprint’. These have been left out of the Code Point polygon dataset and are listed separately in a ‘Discards’ look-up table.

So postcode boundaries are defined, but are synthetic. They are incredibly useful given the extensive utilisation of postcodes, but they are not in not CodePoint Open. However for those in HE they are available through Digimap.