Census results are is available for a range of areas as defined on Census day 2001, or in some cases as changed after the Census. All are built from Output Areas specially created for the release of Census results.
Products include viewable or re-usable boundaries and look up tables providing constitutions of various types of larger area in terms of Output Area 'building bricks'.
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.
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. It was also the basis for the production of improved small areas for the presentation of statistics - the Output Areas.
The 2001 Census Output Areas are designed specifically for statistical purposes. They are based on data from the 2001 Census and are 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 country. 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. The system produced compactly 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 by Professor David Martin of the University of Southampton to provide a detailed account of the methods used in the Output Area production system can be accessed HERE.
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 widely stated requirement in consultation, and also complied with the wider National Statistics policy on the 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...).
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, were built from a 'best fit' of these component building bricks. This is to remove the 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 allocated all records to the part in which the most population fell, as indicated by the co-ordinate references of the records. This process could 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 risks in releasing 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.
Special arrangements were made for wards (electoral divisions in Wales), parishes/communities, and postcode sectors which were too small for the release of the usual output for such areas. More information, and lists of affected areas, is available HERE.
Main products are Output Area boundaries - for use in reference maps, in thematic maps, or in geographical information systems - and directories or look-up tables. These directories show the Output Areas which form higher geographies and the postcodes which form each Output Area. 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.
Viewable maps of boundaries
Viewable maps are available without charge through Neighbourhood Statistics as:
an entry route to the Census results
the basis for pre-defined thematic maps for standard geographies
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 in Neighbourhood Statistics.
The largest scale viewable 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 for named places, through reference maps, and for specific topics is available HERE
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.
Output Area to higher areas look up table
The tables are provided on a CD which contains an Output Area (OA) to higher areas look up file, together with associated look up file names. It allows results such as CAS for Output Areas to be aggregated to higher level areas such as Parliamentary constituencies, urban areas, or postcode sectors. Information is also included for local authority, health and electoral geographies, as are details of sub-threshold areas which have been combined for the purposes of Census output.
A full technical description of the content on the CD ia available HERE (79.2kb PDF file), and a summary of the higher areas is available HERE.
Output Areas to 2004 wards lookup files
A set of Output Area to ward lookup files was released on 18 November 2004 to provide the 'best-fit' of Output Areas to a range of geographies current at the end of 2002. These files supplement the Output Area to higher areas lookup files, released in March 2003.
One file provides the best-fit of Output Areas to 2004 wards (that is, those wards which have undergone a boundary change through a Statutory Instrument made in 2003). A second file provides the best-fit of 2001 Output Areas to the ward boundaries used for the main 1991 census output. Additional files provide more accurate Output Area constitutions of a small number of unchanged 2003 wards in Milton Keynes, Pembrokeshire, Monmouthshire, Lancaster, North Norfolk and South Norfolk where the original constitutions were not based on the final correct ward boundaries.
These lookup files are available, free of charge, as zipped e-mail attachments in text (.txt) or Excel (.xls) formats from Census Customer Services.
The postcode headcounts provide counts (with small cell adjustment) of households, persons, males and females for unit postcodes (including parts of postcodes split by ward or parish boundaries) as at Census Day. The counts are available on a CD containing files in .CSV, .TXT, .DBF and .MDB formats.
The headcounts were provisionally released in Summer 2003 with a warning that some errors were known to exist. These errors were corrected and the counts re-issued in January 2004.
Ward codes and names
A file in Excel format is available from Census Customer Services (220kb PDF file) which provides ward names for use with results in CSV format which contain ward codes but not ward names.
The vector (re-usable) boundaries for the 175,434 Output Areas in England and Wales are available from Census Customer Services (220kb PDF file) for users who wish to set up the boundaries in their own Geographical Information Systems. The boundaries are available either on a DVD or on four CDs.
DVD incorporating all the material on the four CDs listed below.
CD1 contains both high and low resolution Shape format boundaries. The boundaries for all Output Areas in England and Wales are provided in a single file.
CD2 contains high resolution Shape format boundaries, provided in separate files for each local authority. Low resolution boundaries are provided in a single file for England and Wales.
CD3 provides the same information as CD1 but in MID/MIF format.
CD4 provides the same information as CD2 but in MID/MIF format.
In addition to the vector boundaries, the DVD and each CD contain unique identifiers for Output Areas, population weighted and geometric centroids for Output Areas, and area measurements for Output Areas, together with a lookup table of Output Areas by unit postcodes.
The boundaries were initially released in Summer 2003. Whilst the boundaries remain unchanged, they were re-issued in January 2004 to correct some errors in the postcode to Output Area lookup table, and to remove some technical problems with boundary intersections experienced by users of the MID/MIF format files.
The vector boundaries on CD would be supplied only if a requirement continues after current stocks are exhausted.
In general terms, viewable (raster or hard copy) boundaries may be used without restriction and copied subject only to the acknowledgement of Crown copyright and source. Standard Ordnance Survey conditions apply to the copying of underlying topographical maps.
In general terms, re-usable (vector) boundaries may be used on acceptance of the condition of the licence
in any organisation, in the public or private sectors, for internal business purposes
by any individual consumer for their private use
jointly between any licence holders or by a contractor for a licence holder
and may be published to promote public or commercial services provided this is not for financial gain and the use of the boundaries is secondary to the user's services and business activities and is not a service or business activity in itself, but, in Ordnance Survey terms, the boundary data may not be modified, altered, decompiled, reverse engineered, or disassembled beyond what is necessary for permitted use or to extract the Ordnance Survey elements in the data.
Such permitted use would include, for example, joint use of Output Areas boundaries in vector form by a local and health authority to prepare maps of Census results to illustrate a report published in any media covering broader subjects relating to the services provided by the authorities. But it would not include use of Output Areas in vector form in a mapping facility in a package sold for marketing applications, where an additional Ordnance Survey licence would be required.