Irish grid reference system
Irish grid reference system is a system of geographic grid
references used for paper mapping in
Ireland (both Northern Ireland
and the Republic of Ireland). 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.
2 Grid letters
3 Eastings and northings
4 Summary parameters of the Irish Grid coordinate system
7 External links
In general, neither
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
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
In 2001, the
Ordnance Survey of
Ireland and the
Ordnance Survey of
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, close to the western (Co. Roscommon) shore, whose grid reference
is N 000 500. This system is used for high accuracy professional
digital mapping and surveying.
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
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 the six 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 the
Spire of Dublin
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,
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
^ How to Use Map Scales and Grids OSI
Leahy, Derek (September 26, 2008). "Irish Grid Reference System".
Ordnance Survey Ireland : 185 years of innovation in mapping.
Ordnance Survey Ireland. Archived from the original on
September 11, 2009. Retrieved 14 August 2009.
The Irish Grid - A Description of the Co-ordinate Reference System
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 Iri