What are Postcodes
A UK postcode is a means by which the Royal Mail can identify postal delivery areas. There are approximately 1.78 million postcodes in the UK, these cover every single address including those in the Channel Islands and the Isle of Man. There are two types of postcode, large user postcodes are allocated to single addresses receiving at least 500 mail items per day (e.g. business addresses) and small user postcodes which are collections of (usually) adjacent addresses. A single small user postcode may contain up to 100 addresses, but 15 is a more typical number.
Postcode Structure
The postcode is structured hierarchically, supporting 4 levels of geographic unit, Area, District, Sector and Unit Postcode:
| Example | Geographic Unit | Number in UK |
| M | Postcode Area | 124 |
| M13 | Postcode District | 3064 |
| M13 9 | Postcode Sector | 11598 |
| M13 9PL | Unit Postcode | 1.78 million (approx) |
Why Are Postcodes So Important?
Because most people know their own postcode, it is widely used for geographically referencing data, however, probably less than 0.1% of the population would know which 2001 Output Area they lived in. If you are asked to fill in a survey then you are normally asked to provide a postcode, most geographically referenced data uses postcodes. Postcodes can be easily cross referenced with many different types of geography using a look up table.
Limitations of Postcodes
- Postcodes may not be a one to one match with other geographies. Postcode boundaries may straddle ward boundaries, in which case they will have been allocated to more than one zone.
- Postcodes can change due to new building developments, both residential and business i.e. housing estates and business parks within an already densely populated inner city area, may lead to postcode exhaustion, thereby leaving no free postcodes to allocate. Because of the geographical nature of the postcode system it's necessary to consider the wider area and allocate new postcode sectors and postcodes. This also applies to buildings that have been demolished. Areas can also be re-coded and codes can be re-used in a different place after just two years. Continuous monitoring is therefore required to avoid data misallocation.
What Does the Census Have to Do with Postcodes?
Census Output Areas were based on clusters of unit postcodes, however, if a postcode straddled an Electoral Ward or Division it was split between two or more OAs. So, there may be no simple link from Output Area to postcode. However, as stated above, postcodes are of vital use for geographically referencing data, so there needs to be a way to match postcode geography to census geography. House price surveys, surveys of GP attendance, crime statistics and consumer surveys all will use the postcode to record where the subject is located.
How do I convert Postcoded Data?
There is a something called the National Statistics Postcode Directory (NSPD) look up table which is a very large list of all the UK postcodes along with lots of other different types of geography zones.
A web interface to the NSPD has been developed by the CDU, called GeoConvert, it can be used for free by staff and students at UK academic institutions. The GeoConvert web site contains guides on input and output data, downloads of NSPD documentation, worked examples, a glossary of terms and answers to FAQs.
UK Academic staff and students can also download the various raw data releases of the NSPD for free from UKBORDERS. CSV or Microsoft Access formats are available. Unfortunately the NSPD is very large and unwieldy publication (current releases in Microsoft Access format are well in excess of 1 Gigabyte in size).
A very useful resource for new-comers to geographical referencing is the Geo-Refer Project. A number of Learning Objects are avaiable to download which introduce and guide the user on subjects such as 'Address referencing', 'Correctly formatiing UK postcodes' and 'Using geographies from multiple time periods'. A User Profile form is available which will produce a customised tutorial page.
