Irish grid reference system

TheIrish grid reference systemis a system of geographicgrid referencesused for paper mapping inIreland(bothNorthern Irelandand theRepublic of Ireland).The Irish grid partially overlaps theBritish grid,and uses a similar co-ordinate system but with a meridian more suited to its westerly location.

Usage

In general, neitherIrelandnorGreat Britainuseslatitudeorlongitudein describing internal geographic locations. Insteadgrid referencesystems are used for mapping.

The national grid referencing system was devised by theOrdnance Survey,and is heavily used in their survey data, and in maps (whether published by theOrdnance Survey of Ireland,theOrdnance Survey of Northern Irelandor 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

Main article:Irish Transverse Mercator

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 ofUTM,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 inLough Ree,close to the western (Co. Roscommon) shore, whose grid reference isN000500.The ITM system was specified so as to provide precise alignment with modern high-precisionglobal positioning receivers.

Grid letters

Irish grid

The area of Ireland is divided into 25 squares, measuring 100 by 100 km (62 by 62 mi), 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, 3 km (1.9 mi) east, 5 km (3.1 mi) north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a 1 km (0.62 mi) square) through to five (for a 1 m (3 ft 3 in)) square; the most common usage is thesix figure grid reference,employing three digits in each coordinate to determine a 100 m (330 ft) square.

Coordinates may also be given relative to the origin of the entire 500 by 500 km (310 by 310 mi) grid (in the format easting, northing). For example, the location of theSpire of DublinonO'Connell Streetmay 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

References

• Leahy, Derek (September 26, 2008)."Irish Grid Reference System".Ordnance Survey Ireland: 185 years of innovation in mapping.Dublin: Ordnance Survey Ireland. Archived fromthe originalon September 11, 2009.Retrieved14 August2009.

External links

• The Irish Grid - A Description of the Co-ordinate Reference System used in IrelandPDF (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 Refconverts & 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.