The Irish grid reference system is a system of geographic

grid reference A projected coordinate systemalso called a projected coordinate reference system, planar coordinate system, or grid reference systemis a type of spatial reference system that represents locations on Earth using Cartesian coordinate system, Car ...

s used for paper mapping in

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

(both

Northern Ireland Northern Ireland ( ; ) is a Countries of the United Kingdom, part of the United Kingdom in the north-east of the island of Ireland. It has been #Descriptions, variously described as a country, province or region. Northern Ireland shares Repub ...

and the

Republic of Ireland Ireland ( ), also known as the Republic of Ireland (), is a country in Northwestern Europe, north-western Europe consisting of 26 of the 32 Counties of Ireland, counties of the island of Ireland, with a population of about 5.4 million. ...

). Any location in Ireland can be described in terms of its distance from the origin (0, 0), which lies off the southwest coast. 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 north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

uses

latitude In geography, latitude is a geographic coordinate system, geographic 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 t ...

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 lett ...

in describing internal geographic locations. Instead

systems are used for mapping. The national grid referencing system was devised by the

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

, and is heavily used in their survey data, and in maps (whether published by the

Ordnance Survey of Ireland Ordnance Survey Ireland (OSI; ) was the national mapping agency of the Republic of Ireland. It was established on 4 March 2002 as a body corporate. It was the successor to the former Ordnance Survey of Ireland. It and the Ordnance Survey of ...

, 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, 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 Lough Ree (), translated to English as ''King's Lake'' or ''King Lake'', is a lake in the midlands of Republic of Ireland, Ireland, the second of the three major lakes on the River Shannon. Lough Ree is the second largest lake on the Shannon af ...

, close to the western ( Co. Roscommon) 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 :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

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 and 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. {{Authority control Geography of Ireland Maps from Ordnance Survey Geographic coordinate systems Land surveying systems Geodesy