The Irish grid reference system is a system of geographic grid references used for paper mapping in

Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...

(both

Northern Ireland Northern Ireland ( ga, Tuaisceart Éireann ; sco, label=Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. North ...

and the

Republic of Ireland Ireland ( ga, Éire ), also known as the Republic of Ireland (), is a country in north-western Europe consisting of 26 of the 32 Counties of Ireland, counties of the island of Ireland. The capital and largest city is Dublin, on the eastern ...

). The Irish grid partially overlaps the British grid, and uses a similar co-ordinate system but with a meridian more suited to its westerly location.

Usage

In general, neither

nor

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It is ...

uses

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north po ...

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...

in describing internal geographic locations. Instead grid reference systems are used for mapping. The national grid referencing system was devised by the

Ordnance Survey Ordnance Survey (OS) is the national mapping agency for Great Britain. The agency's name indicates its original military purpose (see ordnance and surveying), which was to map Scotland in the wake of the Jacobite rising of 1745. There was ...

, 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.

2001 recasting: the ITM grid

In 2001, the Ordnance Survey of Ireland and the Ordnance Survey of Northern Ireland jointly implemented a new coordinate system for Ireland called Irish Transverse Mercator, or ITM, a location-specific optimisation of

UTM UTM may refer to: Computing * Unified threat management, an approach to network security * Universal Turing machine, a theoretical computer * Urchin Tracking Module, a Web analytics package that served as the base for Google Analytics * Usabil ...

, which runs 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 "Steadfast Irish heart" , image_map = Island of Ireland location map Roscommon.svg , subdivision_type = Country , subdivision_name = Ireland , subdivision_type1 = Province , subdivision_name1 = Connacht , subdi ...

) shore, whose grid reference is . The ITM system was specified so as to provide precise alignment with modern high-precision global positioning receivers.

Grid letters

The area of Ireland is divided into 25 squares, measuring , each identified by a single letter. The squares are numbered A to Z with I being omitted. Seven 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, east, north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a square) through to five (for a ) square; the most common usage is the ''six figure grid reference'', employing three digits in each coordinate to determine a square. Coordinates may also be given relative to the origin of the entire 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

:Spheroid: Airy Modified, :Datum: 1965, :Map projection:

Transverse Mercator The transverse Mercator map projection (TM, TMP) is an adaptation of the standard Mercator projection. The transverse version is widely used in national and international mapping systems around the world, including the Universal Transverse Merc ...

: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

Notes

References

*{{cite web, first=Derek , last=Leahy , title=Irish Grid Reference System , url=http://www.osi.ie/en/alist/reference-information.aspx?article=42177d93-831a-4c6c-8ebf-cef57b55cd93 , work=Ordnance Survey Ireland : 185 years of innovation in mapping , publisher=Ordnance Survey Ireland , location=Dublin , date=September 26, 2008 , access-date=14 August 2009 , ref=OSI , url-status=dead , archive-url=https://web.archive.org/web/20090911204112/http://www.osi.ie/en/alist/reference-information.aspx?article=42177d93-831a-4c6c-8ebf-cef57b55cd93 , archive-date=September 11, 2009

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 references

LatLong <> OSi Grid Ref
converts & presents in many formats, generates specific links to that location for several useful map web pages - 1840 - present. LatLong WSG84 <> GB, Ireland (inc NI) and Chanel Islands (30U) GR formats recognised. Distance measure for dog-leg routes & area calculations. Geography of Ireland Maps from Ordnance Survey Geographic coordinate systems Land surveying systems Geodesy