Output geography
Census results are available for a range of areas, current at the time of the Census, or, in same cases, as changed subsequently.
There are viewable or re-usable boundaries and look up tables providing constitutions of various types of larger area in terms of Output Area 'building bricks'.
Area types
The main types of administrative area recognised by Census output are: local government; health areas; Parliamentary constituencies; urban areas; parishes (Communities in Wales); and postcode sectors. Boundaries may be inconsistent with one another and subject to change, which gives a complex map of overlapping geographies. A summary diagram showing the relationships between area types can be seen HERE (53.5kb pdf file), and a summary of statistical products available by area type can be seen HERE.
Reference maps
Reference maps for the areas used to present Census results are viewable online, with the exception of postal geography which does not have precise boundaries. These can be accessed by clicking the 'i' buttons in the left hand column heading on the entry point. These open a description of an areas type in a glossary, with links to reference maps.
These are of two types.
First, PDFs of reference maps in Census reports (check the 'contents' to find the maps), together with similar PDFs which are part of background information provided by ONS Geography - which generally show the larger areas within the whole country or quite large parts of it, without detailed topographical underlays. The former show boundaries and names as they were at the reference date for the Census results, and the latter show the areas as they are currently.
Second, local views of the boundaries of smaller areas overlain on topographical base maps are available through Neighbourhood Statistics or Nomis, which are very helpful in pin pointing the results for and selecting a local area, and displaying figures for it. There are also maps of the boundaries of larger areas such as local authorities. The 'Customised Tables , Charts, and Maps' option on the Neighbourhood Statistics homepage provides access to the maps, particularly for wards, parishes/communities, and Output Areas, and there is introductory guidance. Nomis offers views of maps, particularly for wards and Output Areas, in both 'wizard' and 'advanced' queries as part of the 'geography' step after activating 'turn maps on'.
Geographical referencing
Census data are aggregated within different boundaries by assembling small geographical 'building bricks' to which the data are coded. The most adaptable and unchanging 'building brick' is a National Grid co-ordinate reference. A grid reference to a resolution of one metre gives an address, and people at it, a unique geographical location. Data with such references can be captured and aggregated for any area with a boundary represented by a line of co-ordinates - termed a digital boundary.
Every record on the output database of the 2001 Census has a co-ordinate reference to a one metre resolution, as well as a postcode and more conventional area codes. This allows the data to be aggregated to any area - current, new or old, or ad hoc, changing or stable. It was also the basis for the production of improved small areas for the presentation of statistics - the Output Areas.
Output Areas
The 2001 Census Output Areas are designed specifically for statistical purposes. They are based on data from the 2001 Census and were built from postcode units. Output Areas are used not only for Census output but also as the basis of Super Output Areas which have been introduced as stable and consistently sized areas for Neighbourhood Statistics.
This contrasts with Censuses from 1961 to 1991 when small area statistics were aggregated and presented for areas drawn up for the conduct of the Census in the field - Enumeration Districts (EDs) - each of which was the workload for an enumerator. The size and character of each ED was determined for operational purposes. It was also convenient to use the ED codes in processing and as the smallest output 'building brick', but variability made them less than ideal for statistical purposes.
Output Area production
The system for creating Output Areas was fully automated, and applied systematic and consistent criteria throughout the England and Wales. Its development became possible through the availability of increased computing power for automatic zoning methods, and also through the availability of co-ordinate referenced and postcoded data based on the Ordnance Survey Address Point product.
In simple terms, the system created Output Areas with around 125 households and populations which tended towards homogeneity. The 175,000 Output Areas 'nest' within wards and parishes, and normally comprise of whole unit postcodes as they were around the time of the Census. The system produced completely shaped areas following natural boundaries where possible, but the underlying patterns of streets and postcodes may result in convoluted shapes.
The system created a notional, computer held boundary around every address with no gaps between - a process known as 'tessellation'. The boundaries internal to each postcode unit (such as PO15 5RR) were removed, leaving a computer held boundary for each postcode. The system then looked for postcodes in contiguous clusters within each ward which optimised the mandatory and desirable criteria for Output Areas, using a process of very rapid iterative combinations. A map illustrating the various components can be viewed HERE, and a Website prepared in 2001 by Professor David Martin of the University of Southampton to provide a detailed account of the methods to be used in the Output Area production system was created.
Output Areas nest within the ward and parish/Community boundaries legally in force at the end of 2002. This includes ward boundaries operative in a number of local authorities in May 2003, and in some others in May 2004, a list of which is available HERE (96kb excel file) as an Excel file. The adoption of boundaries current at the release date of the statistics was a requirement widely supported in consultation, and also complied with a wider National Statistics policy on harmonisation of boundaries. More information on harmonisation and on the number of Output Areas in each local authority is HERE.
Super Output Areas
Larger standard building bricks - known as Super Output Areas - for use in official statistics have been produced outside the 2001 census programme by grouping Output Areas by automated zoning methods. Census Key Statistics and some Census Area Statistics (CAS) tables are available for Super Output Areas [more details...].
Overlapping geographies
Exact Census counts were produced only for a single geography - Output Areas, parishes/Communities and wards, or areas built exactly from them, as at 31.12.2002. Other geographies, with the exception of National Parks, are built from a 'best fit' of these component building bricks. This is to remove a risk that information about identifiable people or households might be disclosed by the differencing of standard sets of statistics for overlapping areas.
The 'best fitting' of a divided Output Area or ward allocates all records to the part in which the most population fell, as indicated by the co-ordinate references of the records. This process can be carried out only with the use of the confidential data held by ONS.
Output for geographies built from best fitting Output Areas is in general limited to the Census Area Statistics (CAS) released at Output Area level. Further consideration may be given to the release of the fuller Standard Tables which are likely to be needed for certain areas, often with large populations, such as health areas (which may not always build from wards), and large aggregates of Output Areas such as 'rural' England.
Sub-threshold areas
Special arrangements were made for wards (electoral divisions in Wales), parishes/Communities, and postcode sectors which had populations too small for the release of the usual output for such areas. More information, and lists of affected areas, is available HERE.
Geographical products Viewable maps of boundaries
Viewable maps are available without charge through Neighbourhood Statistics as:
reference maps
an entry route to the Census results
Local authority, health area, electoral area, ward, parish/community, Output Area and Super Output Area boundaries are viewable at appropriate scales on Ordnance Survey topographical base maps.
The largest scale viewable in Neighbourhood Statistics is the 1:10,000 map which shows many street names, although parts of an Output Area boundary may be off screen at the maximum zoom, particularly in rural areas.
Special guidance on finding Census figures in Neighbourhood Statistics for named places, through reference maps, and for specific topics is available HERE.
Maps of wards and Output Areas boundaries on Ordnance Survey 1:50,000 scale topographical base maps are also viewable through the Nomis service.
Hard copies of Census boundaries overlaid on topographic base maps can be provided as A3 sheets at the cost of reproducing the maps, including royalties to Ordnance Survey, from Census Customer Services.
Vector boundaries
Files of vector (re-usable) boundaries of Output Areas in England and Wales are available, generally without charge, to users who wish to set up the boundaries in their own geographical information systems. Additional software is required to view and use the vector boundaries. [more details...].
Lookup tables
These directories give the Output Areas which form higher geographies and the postcodes which formed each Output Area around the time of the Census. They may be used to aggregate results for Output Areas to higher areas or to link non-census data coded to postcodes to the 'denominators' provided by Census statistics. The products are generally supplied without charge, although material costs may be recovered. [more details...]
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