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Convert Grid Reference points to Latitude/Longitude

In general, neither Ireland nor Great Britain uses latitude or longitude in describing internal geographic locations. Instead grid reference systems are in common usage.

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.

OS Grid References are based on 100km grid squares identified by letter-pairs (in Great Britain) or a single letter (in Ireland), followed by digits which locate a point within the grid square, as explained by the Irish national grid reference system or British national grid reference system. 6-digit references provide precision to 100m grid squares; 8 digits to 10m grid squares, and 10 digits to 1m.

The Ordnance Survey (UK) grid is a Transverse Mercator projection (with origin at 49°N, 2°W) based on the Airy 1830 ellipsoid using the OSGB36 datum. Ordnance Survey of Ireland grid is also a Transverse Mercator projection (with origin at 53°30'N, 8°W) based on a modified Airy 1830 ellipsoid using the OSGB1936 modified datum. Note that GPS is based on WGS84/GRS80, which can vary from OSGB36 by as much as 120m or 6" or arc. The latitude/longitude given here is converted in WGS84.