Area Measurements in Northern Ireland 

Two sets of area measurements exist for administrative areas within Northern Ireland; the major difference between the two estimates relates to the inclusion or otherwise of inland water. EU guidelines state that inland water should be omitted from area measurements, and NISRA has made initial estimates that meet this requirement in a provisional way. However, official local government boundaries delineated by Ordinance Survey Northern Ireland (OSNI) include inland water and Census outputs are based on the official boundaries. This short paper provides details on the two sets of estimates.

 2001 Census based outputs

Outputs from the 2001 Census Output Tables are based on the official local government boundaries as delineated by OSNI; these include inland water and estuaries. An example concerning Lough Neagh is shown below. The 2001 Census estimates use contiguous land and thus exclude major offshore islands.  Further, the Census estimates do not allow for the effect of land terrain.

Lough Neagh

Official 2001 Census Electoral Ward and LGD boundaries include inland water such as Lough Neagh.  This can be seen below where the boundary of Cranfield in Antrim DC extends well into Lough Neagh.

Rathlin Island

The exclusion of major offshore islands resulted in the area for Rathlin Island being discounted from the area measurement in the 2001 Census.  The following map shows the ward of Bonamargy and Rathlin and how it is split into its constituent parts of Ballycastle town and Rathlin Island. 

Although Rathlin Island was excluded for the area measurements, the population of the island and their characteristics was included in Census output.  As part of the creation of Output Areas for the 2001 Census each household point on the island was allocated the postcode of a property in Ballycastle to militate against a risk of disclosure of households or individuals.  This resulted in the creation of Census Output Area (95UU030002) within the ward of Bonamargy and Rathlin that included data pertinent to Rathlin Island along with the relevant part of the town of Ballycastle.  This had an obvious effect on population density calculations as shown in the table below. 

Census geography is hierarchical, with Output Areas aggregating to wards / Super Output Areas, which in turn aggregate to Local Government Districts / Parliamentary constituencies. This simple additivity applies throughout the Census providing consistency throughout the Census results for all hierarchical levels of administrative geography in NI.

EU Guidelines /  NISRA estimates

EU guidelines on the definition of area measurements state that inland water should be excluded.  NISRA has estimated area measurements for Local Government Districts that are broadly consistent with these EU guidelines. These measurements were derived by NISRA from 1:50,000 scale local authority maps supplied by OSNI. 

NISRA area measurements for Local Government Districts are available from the Demography section of the NISRA web site (www.nisra.gov.uk). 

In areas affected by inland water / estuaries, the NISRA area estimates are lower as they exclude inland water and estuaries; this is seen for Antrim, Belfast, Cookstown, Craigavon, Derry, Dungannon, Fermanagh, Magherafelt, Newry & Mourne,

In areas with offshore islands, the NISRA area estimates are higher; this is seen for Ards, Moyle.

In the remaining Local Government Districts, the Census and NISRA estimates are very similar – Armagh, Ballymena, Ballymoney, Banbridge, Carrickfergus, Castlereagh, Coleraine, Larne, Limavady, Lisburn, Newtownabbey, North Down, Omagh, Strabane. Differences between the two estimates for these areas are due to differences in map resolution.

A table of the two sets estimates is provided at the end of this short paper.

One further minor difference relating to Dundrum Bay in Down is noted.

Dundrum Bay

The map below shows the ward of Dundrum and an area of water known as Dundrum Bay.  The map shows that Dundrum Bay is not included within the official ward boundary, therefore the census area measurement for this ward excludes the bay.

The shaded area on the map marks an area that is tidal but landlocked by a bridge carrying a road.  When estimating the area according to EU guidelines, the NISRA process that delineated the area followed the road and thus included this part of an estuary.  The potential effect on population density calculations is shown in the table below.

Summary

The table below lists two separate estimates of the areas of the 26 Local Government Districts in Northern Ireland. The Census figures have the virtue of aggregating exactly in an internally consistent way from Output Area up to Northern Ireland, and match the official local government boundaries. However, the Census figures are not consistent with EU guidelines as they include areas of inland water and estuaries, and exclude off-shore islands. The NISRA figures approximate the EU guidelines but are based on internal NISRA estimates using 1:50,000 resolution maps.

Recommendations

It is recommended that the following footnote be applied when using area measurements from the Census -

The area measurements reported here are based on the official local government boundaries; they include areas of inland water and estuaries. The areas of off-shore islands have been omitted. The estimates do not allow for the effect of land terrain.

It is recommended that the following footnote be applied when using the NISRA area measurements –

The area measurements reported here are based on internal analyses by NISRA of large scale mapping from OSNI. The area measurements approximate EU guidelines through the omission of areas of inland water and estuaries.