Irish grid reference system

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The Irish grid reference system is a system of geographic grid references commonly used in Ireland (both Northern Ireland and the Republic of Ireland). The Irish grid overlaps the British grid.

Usage

In general, neither Ireland nor Great Britain uses latitude or longitude in describing internal geographic locations. Instead grid reference systems are in common usage.

The national grid referencing system was devised by the Ordnance Survey, and is heavily used in their survey data, and in maps (whether published by the Ordnance Survey of Ireland, the Ordnance Survey of Northern Ireland or commercial map producers) based on those surveys. Additionally grid references are commonly quoted in other publications and data sources, such as guide books or government planning documents.

The Ordnance Survey of Ireland has implemented a new coordinate system for Ireland called Irish Transverse Mercator, or ITM, which will initially run in parallel with the existing Irish grid system. In both systems, the true origin is at 53° 30' N, 8° W — a point in Lough Ree, close to the western (Co. Roscommon) shore, whose grid reference is N 000 500.

Grid letters

The area of Ireland is divided into 25 squares, measuring 100 km by 100 km, each identified by a single letter. The squares are numbered A to Z with I being omitted. 7 of the squares do not actually cover any land in Ireland - A, E, K, P, U, Y and Z.

Eastings and northings

Within each square, eastings and northings from the origin (south west corner) of the square are given numerically. For example, G0305 means 'square G, 3 km east, 5 km north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a 1 km square) through to five (for a 1 m square); the most common usage is the six figure grid reference, employing three digits in each coordinate to determine a 100 m square.

Coordinates may also be given relative to the origin of the entire 500 by 500 km grid (in the format easting, northing). For example the location of the Spire of Dublin on O'Connell Street may be given as 315904, 234671 as well as O1590434671. Coordinates in this format must never be truncated, because, for example, 31590, 23467 is also a valid location.

Summary parameters of the Irish Grid coordinate system

Datum: Airy Modified,

Map projection: Transverse Mercator,

Latitude of Origin: 53°30'00 N,

Longitude of Origin: 8°00'00 W,

Scale Factor: 1.000 035,

False Easting: 200000 m,

False Northing: 250000 m

References

• Leahy, Derek (September 26, 2008). "Irish Grid Reference System". Ordnance Survey Ireland : 185 years of innovation in mapping. Dublin: Ordnance Survey Ireland. http://www.osi.ie/en/alist/reference-information.aspx?article=42177d93-831a-4c6c-8ebf-cef57b55cd93. Retrieved 14 August 2009.

Examples

• O 008 741 (Newgrange Neolithic tomb, County Meath on OSNI 1:250000 mapping from OrdnanceSurvey.co.uk Get-a-Map).
• C 948 454 (Giant's Causeway, County Antrim on OSNI 1:250000 mapping ibid).

External links

• The Irish Grid - A Description of the Co-ordinate Reference System used in Ireland -PDF (358kB) on the Irish Grid from OSi.
• PDF (221KB) detailing GPS to Irish Grid conversions
• A detailed (42-page) PDF file including history and map of the Irish grid and its links to Britain
• A two-page article for the American Society for Photogrammetry and Remote Sensing summarising the above.
• History and overview of the system, from OSI
• OSI online coordinate converter tool
• Clickable webmap that shows Irish grid referencesga:Eangach thagartha náisiúnta na hÉireann