Editing Irish grid reference system

{{short description|System of geographic grid references used for mapping in Ireland}}
{{Use Hiberno-English|date=July 2024}}
{{Use dmy dates|date=March 2025}}
{{Geodesy}}
The '''Irish grid reference system''' is a system of geographic [[grid reference]]s used for paper mapping in [[Ireland]] (both [[Northern Ireland]] and the [[Republic of Ireland]]).<ref>[https://www.osi.ie/education/map-reading/how-to-use-map-scales-and-grids/ How to Use Map Scales and Grids] OSI</ref> Any location in Ireland can be described in terms of its distance from the origin (0, 0), which lies off the southwest coast.<ref name="ONS Geography Guide">{{cite web |url=https://geoportal.statistics.gov.uk/datasets/a-beginners-guide-to-uk-geography-2023/about |title=A Beginners Guide to UK Geography (2023) |author=<!--Not stated--> |date=24 August 2023 |website=Open Geography Portal |publisher=Office for National Statistics |access-date=9 December 2023 |quote=The Irish National Grid is a common referencing format for geographic data in Ireland. It covers both Northern Ireland and Eire (the Republic of Ireland). The result is that any location in Ireland can be described in terms of its distance from the origin (0, 0), which lies off the southwest coast.}}</ref> The Irish grid partially overlaps the [[British national grid reference system|British grid]], and uses a similar co-ordinate system but with a meridian more suited to its westerly location.

==Usage==
In general, neither [[Ireland]] nor [[Great Britain]] uses [[latitude]] or [[longitude]] in describing internal geographic locations. Instead [[grid reference]] systems are used for mapping.

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.

==2001 recasting: the ITM grid==
{{main|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  of [[Universal Transverse Mercator coordinate system|UTM]], which runs in parallel with the existing Irish grid system.  In both systems, the true origin is at 53°&nbsp;30'&nbsp;N, [[8th meridian west|8°&nbsp;W]]<ref>{{Cite web|url=https://osi.ie/resources/reference-information-2/irish-grid-reference-system/|title = Irish Grid Reference System}}</ref> — a point in [[Lough Ree]], close to the western ([[County Roscommon|Co. Roscommon]]) shore, whose grid reference is {{Oscoor|N000500}}. The ITM system was specified so as to provide precise alignment with modern high-precision [[Satellite navigation|global positioning receivers]].

==Grid letters==
[[File:Irish Grid.svg|thumb|right|Irish grid]]
The area of Ireland is divided into 25 squares, measuring {{convert|100|by|100|km|abbr=on}}, 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, {{convert|3|km|abbr=on}} east, {{convert|5|km|abbr=on}} north'.  A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a {{convert|1|km|abbr=on}} square) through to five (for a {{convert|1|m|abbr=on}}) square; the most common usage is the ''six figure grid reference'', employing three digits in each coordinate to determine a {{convert|100|m|abbr=on}} square.

Coordinates may also be given relative to the origin of the entire {{convert|500|by|500|km|abbr=on}} 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: [[8th meridian west|8°00'00 W]]
:Scale factor: 1.000 035
:False easting: 200000 m
:False northing: 250000 m

==Notes==
{{reflist}}

==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=26 September 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=11 September 2009 }}

==External links==
* [https://www.osi.ie/wp-content/uploads/2015/04/The-Irish-Grid-A-Description-of-the-Coordinate-Reference-System-Used-in-Ireland.pdf ''The Irish Grid - A Description of the Co-ordinate Reference System used in Ireland''] PDF (358kB) on the Irish Grid from OSi.
* [https://www.osi.ie/wp-content/uploads/2015/05/transformations_booklet.pdf PDF (221KB) detailing GPS to Irish Grid conversions]
* [https://web.archive.org/web/20120204092951/http://www.osni.gov.uk/2.1_the_irish_grid.pdf A detailed (42-page) PDF file including history and map of the Irish grid and its links to Britain]
* [https://web.archive.org/web/20070701073911/http://www.asprs.org/resources/grids/03-99-ireland.pdf A two-page article for the American Society for Photogrammetry and Remote Sensing summarising the above.]
* [https://web.archive.org/web/20090911204112/http://www.osi.ie/en/alist/reference-information.aspx?article=42177d93-831a-4c6c-8ebf-cef57b55cd93 History and overview of the system, from OSI]
* [https://www.osi.ie/services/geodetic-services/coordinate-converter/ OSI online coordinate converter tool]
* [http://www.gridreference.ie Clickable webmap that shows Irish grid references]
* [http://osgr.mister.red 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.

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[[Category:Geography of Ireland]]
[[Category:Maps from Ordnance Survey]]
[[Category:Geographic coordinate systems]]
[[Category:Land surveying systems]]
[[Category:Geodesy]]