For Census products, the definition of an urban area is an extent of at least 20 hectares and at least 1,500 residents at the time of the 2001 Census. The starting point is the identification by Ordnance Survey of areas with land use which is irreversibly urban in character. This comprises
• permanent structures and the land on which they are situated, including land enclosed by or closely associated with such structures;
• transportation corridors such as roads, railways and canals which have built up land on one or both sides, or which link built-up sites which are less than 200 metres apart;
• transportation features such as airports and operational airfields, railway yards, motorway service areas and car parks;
• mine buildings, excluding mineral workings and quarries; and
• any area completely surrounded by built-up sites."
Areas such as playing fields and golf courses are excluded unless completely surrounded by built-up sites.
The prerequisite for the recognition of an urban area is that the area of urban land should extend for 20 hectares or more. Separate areas of urban land are linked if less than 200 metres apart. Land between built-up areas is not regarded as urban unless it satisfies one of the conditions listed above. The critical factor for the inclusion of an urban area in Census products is a minimum population of 1,500 residents at Census day in the Output Areas - the smallest building bricks for which detailed Census statistics are released - which best fit to the boundary of the urban land. Areas of urban land of at least 20 hectares which do not meet this minimum population threshold are not treated as urban areas in Census output, but as settlements with fewer than 1,500 people.
The second stage was therefore to fit Output Areas to each area of urban land, and to calculate the combined population of the Output Areas falling within each urban area. The automated procedure allocated an Output Area to an area of urban land where it fell entirely within the land or when the majority or largest proportion of the population located by the one metre coordinate references of addresses fell within the land. This produced a good fit of population to the urban land, but separated the extent of urban areas. The use of a ‘best fit’ definition follows the practice used to define urban areas for the 1981 and 1991 reports, but also complies with National Statistics policy on the general use of stable small area building bricks and meets confidentiality, or disclosure protection, requirements. The release of statistics for the exact boundaries of urban land could pose the risk that information about small numbers of people could be deduced by the comparison of standard sets of statistics for overlapping areas, but the risk is avoided by the use of standard building bricks.
Urban area digital boundaries are available on CD-Rom from the Office for National Statistics. The boundary files are low resolution (generalised to a tolerance of 20 metres, with intermediate points being removed within this tolerance if they did not significantly change the angle of the line), mapped to the Extent of the Realm (the full administrative extent, in many cases, the Mean Low Water Mark). Low resolution files are suitable for use in conjunction with Ordnance Survey raster map products, for producing detailed regional and local maps, or large wall maps and also for non-demanding GIS analyses (such as buffering). Attribute data supporting teh boundaries is limited to area codes and names.
Boundaries are provided for both urban areas ans settlements with fewer than 1,500 people.
The files are available in both ESRI Shapefile format and MapInfo MID/MIF format.