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Using Geographies from Multiple Time Periods

Not only are social science data collected and reported for very many different geographical units, but these geographies are subject to extensive change over time. Changes may range from minor realignment of boundaries, through renaming or recoding of areas, to wholesale redesign. Very minor adjustments may not require any reassignment of data between geographical units but it is sometimes the case that changes between time periods are so great that it is necessary to treat two "generations" of the same geography as if they were entirely separate systems. The existence of a geographical unit with a given name or code at two separate times (e.g. "Portswood ward") should never be treated as automatic evidence that the same geographical area is being referenced. Temporal changes are frequently scale-dependent, for example in the case of electoral wards, where a boundary review may result in ward-level boundaries being redesigned to accommodate population changes, within a local authority boundary which remains unchanged at that time.

The major geographical referencing systems in the UK are revised with widely differing frequencies, driven by the primary purposes for which they were created. Four major geography systems with quite different change processes are outlined in the table:

Geography Frequency of change Type of change
Census output geography
Once per decade Until 2001, Census geographies were frozen versions of the administrative geography of local government wards and parishes in place at the date of the census (or for 2001, at the date of census data publication in 2003). Since 1971 these have been subdivided by a further tier of small areas (variously enumeration districts or output areas) unique to each census. In 2011, the Office for National Statistics has made an important change to this principle by allowing the small area census geography to reflect the hierarchy of super output areas used for Neighbourhood Statistics rather than nesting neatly within wards. In Scotland, considerable effort has been devoted to maintaining intercensal comparability between areas, whereas changes in the ward-level geography have caused change in around two thirds of the smallest areas between censuses elsewhere in the UK.
Postcode geography
Continuous change The postcode system is maintained for the purpose of delivering mail and must therefore be adaptive to the needs of the postal delivery system and to residential demolition and development. Postcodes are created, changed and discontinued on a continuous basis. Discontinued postcodes may also be reallocated although such reuse is rarely immediate and can be traced by examining successive postcode directories. The system is broadly comparable across the entire UK.
Administrative geography
Periodic revision The various tiers which comprise regional and local government are static for long periods and then subject to major revisions. The system of local authorities created in 1974 in England and Wales remained largely stable until the mid-1990s when several rounds of redefinition created a new structure, including the loss of county identities such as "Avon" and the creation of two-tier local government in some areas of England and single-tier unitary authorities in others. The detailed structure is different in each country of the UK.
Neighbourhood statistics geography
Once per decade The Neighbourhood Statistics geography hierarchy of Output Areas (OAs), Lower Layer Super Output Areas (LSOAs) and Middle Layer Super Output Areas (MSOAs) was first created following the 2001 Census and shared the OAs with the census geography. No changes were made to these units during the decade, but they have been updated in preparation for publication of 2011 Census outputs, effectively becoming the principal 2011 Census geography.

Electoral and health geographies are especially complex and are subject to frequent and substantial revisions, reflecting the needs of electoral representation and health care organization. These can be particularly difficult to trace due to the fact that revisions often take the form of piecemeal boundary adjustments and mergers of local areas without consistent treatment of area names and codes. Further, periodic major reorganizations of the health care system leads to major discontinuities, for example the discontinuation at various dates of district health authorities (DHAs), regional health authorities (RHAs), family health service authorities (FHSAs), primary care trusts (PCTs) and Strategic Health Authorities (SHAs) in England. Again, there area differences in health geography between the countries of the UK.

Changes in the principal administrative geographies of the UK have been recorded in the ONS Postcode Directory (ONSPD) and preceding postcode directories, which permit a good degree of matching in some cases and include matches to more than one recent health geography. Many new fields were added to these directories in the early 2000s and data for different area codes can be matched using database software such as Microsoft Access. The GeoConvert facility will match data between geography codes using postcode directory data from 2006.

Very little information exists about the detailed relationship between small areas in the UK before the 1980s. Where no lookup table information is available to describe the relationship between two geographies, geographical information systems (GIS) software can be used to intersect two sets of boundaries and create records for the new areas which are created - for example by intersecting ward boundaries from two different time periods. This is not a task that would usually be undertaken by a novice user, and digital boundary data do not always exist in these cases where no lookup tables are available. Further, the intersection of two sets of area boundaries can show which areas overlap but does not provide any direct estimate of the numbers of persons or households contained in each intersecting area, which is usually the issue of interest to the user. The question of how best to allocate information between areas - known as areal interpolation - has been the subject of extensive research, but these methods cannot accurately recreate detailed information which was never collected at the time.

Additional Resources

The ONS Beginners Guide to UK Geography contains useful overviews of the numerous geographical boundary systems in use [http://www.ons.gov.uk/ons/guide-method/geography/beginner-s-guide/index.html]

Successive revisions of the National Statistics Postcode Directory (NSPD) trace many of the principal boundary changes [ http://www.ons.gov.uk/ons/guide-method/geography/products/postcode-directories/-nspp-/index.html]