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Working with boundaries in our Web services

Our data, derived from Boundary-Line, comprises administrative and electoral boundaries for England, Scotland and Wales. These include counties, unitary authorities, Westminster constituencies, wards and many more. They have been derived from Boundary-Line to ensure compatibility and good performance with web applications built using OS OpenSpace API. The geometry information is represented as vector digital data, supplied in GeoJSON format.

With our data derived from the Boundary-Line product you can:

  • Customise and style boundaries for your web site.
  • Design thematic maps using your own or third party data.
  • Inspire and amaze people with environmental, political and other social statistics.
  • Experiment and innovate.

High level administration relationships in our data:

In England
A County (CTY) is divided into Districts (DIS) which is divided into District Wards (DIW). For voting purposes a County is divided into Electoral Divisions (CED)

Unitary Authorities (UTA) are, for voting purposes, divided into Unitary Authority Wards (UTW) - except in the Isle of Wight, Cornwall, County Durham, Northumberland, Shropshire and Wiltshire where it is the Unitary Authority Electoral Division (UTE)

England also has Metropolitan Districts (MTD); these have no County administration. These Districts are divided for voting purposes into Metropolitan District Wards (MTW).

With the introduction of the regional government for London, in this document called the Greater London Authority (GLA); the proportional representation voting area for this new administration is the Greater London Authority Assembly Constituency (LAC). The existing London Borough (LBO) structure also has a relationship with the new Greater London Authority and each London Borough is divided for voting purposes into London Borough Wards (LBW).

In Scotland
Unitary Authorities (UTA) are, for voting purposes, divided into Unitary Authority Wards (UTW).

The Scottish Parliamentary Electoral Region (SPE) is divided into Scottish Parliamentary Constituencies (SPC). Scotland is divided into Parliamentary Constituencies for voting purposes; to elect Members of the Scottish Parliament.

In Wales
Unitary Authorities (UTA) are, for voting purposes, divided into Unitary Authority Electoral Divisions (UTE).

The Welsh Assembly Electoral Region (WAE) is divided into Welsh Assembly Constituencies (WAC). Wales is divided into Welsh Assembly Constituencies for voting purposes; to elect Assembly Members to the Welsh Assembly.

In Great Britain
Great Britain is divided into European Regions (EUR) (Scotland, Wales and nine English regions) for voting purposes to elect members to the European Parliament.

England, Scotland, and Wales are divided into Westminster Constituencies (WMC) for voting purposes to elect members to the Westminster Parliament. Westminster Constituencies are made up of combinations of Wards (or Unitary Authority Electoral Divisions (UTE) in Wales), but they don't have consistent common boundaries.

The following list shows OS OpenSpace boundary DESCRIPTION, (AREA_CODE) and feature count (number of boundary areas of this type)

  • County, (CTY) 27
  • County Electoral Division, (CED) 1739
  • District, (DIS) 201
  • District Ward, (DIW) 4585
  • European Region, (EUR) 11
  • Greater London Authority, (GLA) 1
  • Greater London Authority Assembly Constituency, (LAC) 14
  • London Borough, (LBO) 33
  • London Borough Ward, (LBW) 649
  • Metropolitan District, (MTD) 36
  • Metropolitan District Ward, (MTW) 815
  • Scottish Parliament Electoral Region, (SPE) 8
  • Scottish Parliament Constituency, (SPC) 73
  • Unitary Authority, (UTA) 110
  • Unitary Authority Electoral Division, (UTE) 1334
  • Unitary Authority Ward, (UTW) 1464
  • Welsh Assembly Electoral Region, (WAE) 5
  • Welsh Assembly Constituency, (WAC) 40
  • Westminster Constituency, (WMC) 632

Our data shows the following characteristics:

Extent of the Realm (EOR)

This is where a boundary area is digitised on the alignment of the mean low water for the seaward extent. This is prevalent particularly around the estuaries of Great Britain. Our data includes the EOR extent.

Area Values
The area values attribute (HECTARES) quoted is retained from the source data. Area values of each ADMIN_UNIT_ID. Our data includes the EOR extent.

Treatment of boundaries in estuaries
The extent of the realm (EOR) follows mean low water and crosses the estuaries at the level of the sea at low water. In this example showing Westminster Constituencies, an adjoining boundary also extends into the estuary.

Seaward extensions
These are artificial extensions to the realm. They extend the local government and parliamentary areas seaward.

Structures in the sea
Breakwaters are structures of wood, stone, metal or other material built to break the force of waves. They can be separate from, or joined to, the mainland. If joined, they are generally included in the local government and parliamentary areas. If separate, they are not part of the local government and parliamentary area unless they are placed therein by legislation of some kind, for example, Plymouth breakwater is included in the local government area whilst the centre portion of the Portland Harbour breakwater is not.

Permanent or solid structures in the sea, such as the forts in the Solent off Portsmouth Harbour and Brighton Marina, are usually included in local government and parliamentary areas.

Islands and rocks in the sea
The following rules govern the representation in Boundary-Line of offshore islands and rocks in the sea:

  • If an offshore island or rock is shown, it will be contained within all the relevant administrative unit collections.

In this example, the Islands of Flat Holm and Steep Holm in the Mouth of the river Severn are contained within the Bristol North West Westminster Constituency.

Our data derived from the Boundary-Line™ product are made up of boundaries are made up of 11,777 individual boundary areas known as features. Each feature has a unique ADMIN_UNIT_ID is made up of the following attributes:

  • Boundary Line Name (NAME)
  • Administrative boundary area (AREA_CODE)
  • Census code (CENSUS_CODE)
  • Geometry as XMIN YMIN XMAX YMAX (BOUNDING_BOX) This geometry information attribute is optional.
  • Centroid of the feature as easting and northing as x,y (CENTROID)
  • Resolution of data (RESOLUTION)
  • Description of the boundary type (DESCRIPTION)
  • Administrative Unit ID (ADMIN_UNIT_ID)
  • Area of the feature in Hectares (HECTARES)
  • Type Code (TYPE_CODE)
  • Type Definition (TYPE DESCRIPTION)

A lookup table (csv) is available to download from the forum with the following attributes:


The NAME is the distinctive name of the boundary area.

The CENSUS_CODE data is created by the Office of National Statistics allows you to link Our data can be used to identify a logical structure between certain boundary areas. For example District Wards nests into Districts which nests into Counties:

  • County (CTY) - Hampshire, 24
  • District (DIS) - Basingstoke and Deane District, 24UB
  • District Ward (DIW) - Basing Ward, 24UBJX

The census codes are present as attributes for the following AREA_CODEs:

  • The ADMIN_UNIT_ID is a unique identifier created by Our.
  • The AREA_CODE is the name of the feature area type e.g. CTY
  • The DESCRIPTION is the type description of the AREA_CODE e.g. County.

The CENTROID is an x,y nominal centre coordinate of the feature

Product resolutions
Our data has been generalised at six different resolutions (RESOLUTION) to ensure that the vector data is appropriately scaled and maintains performance at different zoom levels.

The following data resolution by zoom level is detailed in a table (csv) in the forum and shows the resolutions available for all boundary areas by AREA_CODE.

Zoom Level

Raster data shown boundaries


10 & 9



8 & 7

1:50 000


6 & 5

1:250 000


4 & 3

MiniScale (1:1M)











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© Ordnance Survey 2019