ONS coding system

In the United Kingdom, the Office for National Statistics maintains a series of codes to represent a wide range of geographical areas of the UK, for use in tabulating census and other statistical data. These codes are referred to as ONS codes or GSS codes referring to the Government Statistical Service of which ONS is part.

The previous hierarchical system of codes has been replaced as from January 2011 by a nine-character code for all types of geography, in which there is no relation between the code for a lower-tier area and the corresponding parent area. The ONS intends to maintain the older coding system in parallel with the new one until the end of 2013.

Information from the 2011 Census is published for a wide variety of geographical units. These areas include:

Super Output Areas (SOAs) are a set of geographical areas developed following the 2001 census, initially to facilitate the calculation of the Indices of Deprivation 2004 and subsequently for a range of additional Neighbourhood Statistics (NeSS). The aim was to produce a set of areas of consistent size, whose boundaries would not change (unlike electoral wards), suitable for the publication of data such as the Indices of Deprivation. They are an aggregation of adjacent Output Areas with similar social characteristics. Lower Layer Super Output Areas (LSOAs) typically contain 4 to 6 OAs with a population of around 1500. Middle Layer Super Output Areas (MSOAs) on average have a population of 7,200. The hierarchy of Output Areas and the two tiers of Super Output Areas have become known as the Neighbourhood Statistics Geography.

The older ONS code was constructed top down:

For example, 12 for Cambridgeshire.

For example, 12UB for Cambridge district or 12UD for Fenland.

For example, 00AL for Greenwich (London Borough) or 00EC for Middlesbrough.

For example, 12UBGA for Petersfield Ward within Cambridge district.

The current system replaces these codes with a fixed length code of nine characters. The first three characters indicate the level of geography and the six digits following define the individual unit. For example, the Royal Borough of Greenwich is coded as E09000011, Middlesbrough is E06000002, Cambridge E07000008 and Fenland E07000010.

...
Wikipedia