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grid reference In Great Britain, a map reference system, overprinted on all Ordnance Survey (OS) maps, which can describe uniquely the location of a point on the ground, on the smallest-scale maps to within 10 m. From a reference point to the west of the Isles of Scilly, the National Grid divides Great Britain (Ireland is excluded) into squares of 100 × 100 km, each designated by two letters. These squares are subdivided into 1 × 1 km squares whose western and southern margins are numbered, and on the smallest-scale maps 100 × 100 m squares are also used. To describe a particular location, the grid reference begins with the identifying letters of the relevant 100 km square (e.g. SH), followed by two digits (e.g. 60) to identify the western edge of the 1 km square and a third digit (e.g. 9) as an estimate of the distance of the point from the western margin of that square. This procedure is called ‘easting’. ‘Northing’ is similar, starting with two digits for the southern edge of the 1 km square (e.g. 54) and a third (e.g. 3) as an estimate of the distance from the southern edge. The full grid reference, consisting of two letters and six digits locates the point to within 100 m (SH 609543 is the grid reference for the summit station of the Snowdon Mountain Railway). On smaller-scale maps, two additional digits, one added to the easting and one to the northing, locate the point to within 10 m.