The National Grid provides a unique reference system, which can be applied to all Ordnance Survey maps of Great Britain, at all scales. Great Britain is covered by 100 kilometre grid squares, each grid square is identified by two letters. On Ordnance Survey maps these squares are further divided into smaller squares by grid lines representing 10 kilometre spacing, each numbered from 0 to 9 from the south-west corner, in an easterly (left to right) and northerly (upwards) direction, as shown on our Guide to the National Grid. The National Grid is designed to cover the whole of Great Britain and extends 700 km to the east and 1300 km to the north of the false origin. Within this framework we can use a National Grid reference to state the position of any point in Britain, to any level of precision that we require. Essentially, we need to state two numbers: an easting (along the horizontal axis) and a northing (along the vertical axis). In 1919 ‘British Grid System’ was adopted on military maps and in 1927 it was replaced by the ‘Modified British System’, which remained in use throughout World War II. In the military grid, areas were broken down into progressively smaller squares, with sides in turn representing 500 km, 100 km, 10 km, 1 km. Letters were allocated to the 500 km and 100 km squares and numbers to the 10 km squares, so that a point of reference could be given in letters and numbers. This grid was in use in World War II. In 1929, a grid with lines 5 000 yards apart and giving full coordinates was authorised and printed on the Fifth Edition of the one-inch map. This series commenced publication in 1931, but together with other experiments the grid was over taken by the Davidson Committee Report of 1938. After reviewing the historical precedents for the use of grids on our maps and assessing their practical advantages a clear recommendation was made that ‘… a National Grid should be superimposed on all large scale plans and on smaller scale maps, to provide one reference system for the maps of the whole country’. At the same time the international metre was put forward as the unit on which the grid should be based. The True origin latitude and longitude coordinates of the National Grid are 49 degrees north: 2 degrees west. The False origin latitude and longitude or coordinates are 49 degrees 45 minutes and 58 seconds north : 7 degrees 33 minutes 23 seconds west. The False origin which lies slightly southwest of the Isles of Scilly was devised to ensure that all National Grid coordinates were positive (for example, to the east and north of origin 00) 400 km are added to all eastings coordinates and 100 km subtracted from all northings coordinates. If coordinates were calculated from the True origin, the positions lying west of the central meridian would be negative and the northings, although positive would exceed 1 000 km for some points in northern Scotland. Our online OS getamap™ service enables you to search for a location, place name or postcode. The grid reference at the centre of the screen is displayed on the top left hand corner of the map. Source.