# Why are there differences in boundaries?

## Two boundary data sets?

Boundaries are shown in two OS products:

* Boundary-Line​
* OS MasterMap Topography Layer

Boundaries have been shown on Ordnance Survey Mapping for over 100 years. These were then digitised as lines into Land-Line from the 1970s and subsequently into OS MasterMap Topography Layer since 2001. They were captured and maintained at large-scale (1:1250/1:2500 scale).

<figure><img src="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-0c3601c2e97406c48b298fea622425ce49848c57%2Fhistoric_data_boundaries.png?alt=media" alt="Historic Ordnance Survey map showing boundary data on the map and in the marginalia" width="563"><figcaption><p>Historic Ordnance Survey map showing boundary data on the map and in the marginalia</p></figcaption></figure>

Since the mid 1990s Boundary-Line has existed as an authoritative polygonised dataset of political and administrative boundaries initially captured and maintained at 1:10000 scale although more recently being maintained to large-scale data.

Boundary-Line shows only boundaries that are Operative and is the authoritative dataset for boundaries in Great Britain. OS MasterMap Topography Layer may show boundaries before they are Operative, or there may be a slight delay before being updated after they are Operative.

## Who is responsible for the boundaries?

In Great Britain there are three national Boundary Commissions, one each with responsibility for England (BCE), Wales (BCW) & Scotland (BCS). These are responsible for UK Parliamentary constituency boundaries.

Local government electoral arrangements are reviewed by the Local Government Boundary Commission for England, the Local Democracy and Boundary Commission for Wales and Boundaries Scotland. These include making changes to wards and electoral districts. LDBCW and Boundaries Scotland also look after the Scottish and Welsh devolved parliaments. Local Authorities are responsible for Parish and Community boundaries.

The outputs of these organisations (changes to boundaries published in Statutory Instruments, Boundary Orders and maps) all then come to Ordnance Survey to be included into Boundary-Line and OSMasterMap Topography Layer

## Data Comparison

{% tabs %}
{% tab title="Boundary-Line" %}

* The authoritative boundaries dataset for Great Britain
* Initially captured at 10:000 scale, currently maintained against larger-scale OS MasterMap data
* Contains Administrative and Political Boundaries
* All data is polygonised and themed by boundary type (County, Districts, Unitary Authorities, Parishes, Westminster Constituencies, and so on.)
* All polygons include attribution such as name, type and links to ONS codes for the different boundaries
* Published twice a year in May and October
* Available as Open Data for anyone to use
* More detail can be found on the [Boundary-Line pages of the OS website](https://www.ordnancesurvey.co.uk/business-government/tools-support/boundaryline-support)

<figure><img src="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-65dd1c6b934df4dacfcabcb1b3a4ca21a663af5b%2Fboundary-line-parishes.png?alt=media" alt="map of Boundary-Line Parish dataset" width="563"><figcaption><p>Boundary-Line Parish dataset</p></figcaption></figure>
{% endtab %}

{% tab title="OS MasterMap Topography Layer" %}

* Captured at large scale (1:1250 - urban, 1:2500 - rural or 1:10,000 – mountain and moorland)
* Contains administrative and political boundaries
* Five different themed/coloured lines are used. There is a hierarchy so that the most important boundary is shown by its symbol and all other coincident boundaries are described by text descriptions. The hierarchy and five lines used are County, District/Unitary Authority, Parish, Parliamentary and Electoral District/Ward.
* Lines contain limited attribution, and nothing about the boundary feature they define, such as name, ONS codes, and so on
* Polygons can be created from these lines by a user, however Mean Low Water would also be required to complete polygons that have a coastal boundary
* Contains historic details of what the boundaries are mered to (1.2m Root of Hedge (RH), Face of Wall (FW), Centre of Road (CR), and so on) and where that mereing changes. Since 2002 any new or amended boundaries are not mered and it's assumed that their alignment is the centre of the underlying topography detail.
* Maintained within 1 year of the Operative date of the boundary and published as part of the 6-weekly cycle of OS MasterMap Topography Layer releases
* Available as part of OS MasterMap Topography Layer
* More detail can be found on the [OS MasterMap Userguide](https://www.ordnancesurvey.co.uk/documents/os-mastermap-topography-layer-product-guide.pdf)

<figure><img src="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-7a9b3592cdcce236936bd7dfac9b9a335176b182%2Ftopography-layer-boundaries.png?alt=media" alt="map of OS MasterMap boundaries" width="563"><figcaption><p>OS MasterMap boundaries</p></figcaption></figure>
{% endtab %}
{% endtabs %}

## Why are the 2 products different sometimes?

The main reason for differences between boundaries in Boundary-Line and OS MasterMap Topography Layer is to do with the scale they were captured at:

* “*Boundary-Line for England and Wales was initially digitised from Ordnance Survey’s boundary record sheets at 1:10 000 scale (or, in some cases, at 1:10 560 scale).*” OS Website
* “*OS MasterMap Topography Layer provides the most detailed and accurate large-scale representation of Great Britain available from Ordnance Survey.*“ – OS MasterMap Topography Layer Technical specification document

As these show, Boundary-Line was initially captured at a smaller scale than OS MasterMap Topography Layer. Boundary-Line is still the authoritative dataset and although these slight differences exist in places, they are normally no more than a few meters and addresses (and therefore citizens) should not fall inside the wrong Ward, County, Parish, etc.

<figure><img src="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-4aebd34dc218c08009bbc9089d912ea5bd7ed623%2Fboundaries-differences.png?alt=media" alt="Map of Boundary-Line and OS MasterMap Topography Layer showing a typical size of discrepancy" width="563"><figcaption><p>Boundary-Line (in black) and OS MasterMap Topography Layer (in pink) showing a typical size of discrepancy</p></figcaption></figure>

## Which boundary data should I use?

<table data-view="cards"><thead><tr><th></th><th></th><th></th><th data-hidden data-card-cover data-type="image">Cover image</th></tr></thead><tbody><tr><td>I want to understand the percentage of woodland by Local Authority – use Boundary-Line</td><td></td><td></td><td data-object-fit="cover" data-alt="thematic map of Scotland"><a href="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-0beca0ae5e53351a7e374dc6cab155ae72534020%2Fboundary-line-uses-choropleth.png?alt=media">boundary-line-uses-choropleth.png</a></td></tr><tr><td>I want to assess addresses per Ward – use Boundary-Line</td><td></td><td></td><td data-object-fit="cover" data-alt="map of addresses in a Ward"><a href="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-798963d622056a3cb549ef2e0a1f0e1caed198de%2Fboundary-line-uses-point-in-polygon.png?alt=media">boundary-line-uses-point-in-polygon.png</a></td></tr><tr><td>I want to match statistics to boundaries for a choropleth map – use Boundary-Line</td><td></td><td></td><td data-object-fit="cover" data-alt="choropleth map of great britain with election results"><a href="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-f7e83c397712252d650e83055f2d8b031db3db5f%2Fboundary-line-uses-choropleth-2.png?alt=media">boundary-line-uses-choropleth-2.png</a></td></tr><tr><td>I want to report incidents (pot-holes, fly-tipping, etc.) by Ward – use Boundary-Line</td><td></td><td></td><td data-object-fit="cover" data-alt="map of incidents"><a href="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-dc18b8639434b0f29f99be9c39f65ca980122fa1%2Fboundary-line-uses-reporting.png?alt=media">boundary-line-uses-reporting.png</a></td></tr><tr><td>I want to understand what a boundary is linked to on the ground – use OS MasterMap Topography Layer</td><td></td><td></td><td data-object-fit="cover" data-alt="map of OS MasterMap boundaries"><a href="https://3774974716-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FzIfdYInJITdcxaLLMhlD%2Fuploads%2Fgit-blob-793e1e7009dc49e8fb3aa3cfbc943ceb8e1dc914%2Fboundary-line-uses-topography.png?alt=media">boundary-line-uses-topography.png</a></td></tr></tbody></table>

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This content has been developed from what was originally a Lightning Talk PowerPoint slide set. These slides are available to PSGA members to view and download from the [PSGA members area of the OS website](https://auth.ordnancesurvey.co.uk/my.policy)