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Scottish Settlements Urban and Rural Areas in Scotland

Annex A - The General Register Office for Scotland Postcode Database and the 2001 Census

The postcode database

1. The General Register Office for Scotland (GROS) maintain digital boundaries and 'index files' of postcodes. Royal Mail distinguish 'small user' and 'large user' postcodes, the latter typically being a single address that receives more than a given number of items of mail a day. GROS draws a boundary for each small user postcode to enclose all of the addresses that Royal Mail has assigned to the postcode. Each large user postcode is located and linked to the small user in whose boundary it falls. The index files link:

• each large user postcode to a small user postcode; and
• each small user postcode to a range of 'higher areas' such as council area, health board, electoral ward, civil parish, Output Area as used for the 1991 Census and so on.

Each small user postcode is given a 'centroid' as well as a boundary. The centroid is chosen as the building nearest the centre of the populated part of the postcode.

2. Where the addresses belonging to a postcode belong to more than one council area the postcode is split and each portion treated as a postcode in its own right. Thus postcodes, and any areas built up from postcodes, can 'nest' exactly into council areas. This is not the case for other area types, such as settlements, where each postcode is wholly allocated to the higher area in which the 'centroid' of the postcode falls.

3. The set of polygons for small user postcodes covers the entire land mass of Scotland to the boundaries. The combined external boundary extends as far as the mean high water springs (MHWS) shown on Ordnance Survey mapping and follows the outline of quays and jetties. This excludes the foreshore areas.