Wednesday, 3 June 2009

What do you know about postcodes?

There is a lot to know about UK postcodes! Here are some interesting key facts:

  • There are around 2 million postcodes in the UK, covering something like 29 million addresses.

  • A postcode is made up of four constituent parts. Take the postcode EH9 1PR as an example:

    • The postcode Area is indicated by the initial one or two letters: EH
    • The post District is indicated by the one or two numbers following the Area: EH9
    • The postcode Sector is indicated by the number after the District: EH9 1
    • The postcode Unit is indicated by the full postcode: EH9 1PR

  • Some buildings can contain delivery addresses with different postcodes. High-rise residential buildings are a good example of these. As far as the Code-Point dataset is concerned these are called Vertical Streets, and their boundaries are recorded as regular squares. These are clearly visible when the postcode boundaries are mapped.

  • Usually postcodes cover around 15 delivery addresses, although this isn't a hard and fast rule.

  • Some postcodes apply to only one address or building. Ordnance Survey's postcode is SO16 4GU. Buckingham Palace also has it's own postcode, SW1A 1AA, and 10 Downing Street's postcode is SW1A 2AA. The House of Commons is SW1A 0AA

  • Postcode unit boundaries (which form the OS Code-Point Polygons dataset) are mathematically calculated around a centroid and are not based on physical boundaries visible on the ground. This centroid is calculated using the coordinates for the delivery points in each postcode. These are not necessarily the precise geographic coordinats of the letterbox in your front door!

  • The Code-Point dataset offered through Digimap provides much more information than just the postcode and the coordinates of it's centroid. Additional information includes the NHS Region code, Ward name, how many delivery addresses are in that postcode, how many of those are domestic or business delivery points. The range of information given is different for Scotland from England and Wales.

  • These enable you to link additional information to the postcode if it is already associated with a code common to the postcode dataset e.g. you can identify which postcodes are associated with data you have collected using Ward codes.

  • You can use Digimap's Postcode Query facility (in the Ordnance Survey Collection) to look up the additional information about a particular postcode.

  • You can use Digimap's Boundary Download facility to download the Code-Point data and the associated polygons.

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