How the National Grid works
The National Grid breaks Great Britain down into progressively smaller squares identified first by letters and then numbers.
The largest unit of the grid is 500km squares each designated by a prefix letter alphabetically from A-Z omitting I - the first letter to be quoted in today's National Grid Reference. Great Britain is covered only by four of these squares: H, N, S and T.
The 500km squares are then further broken down in to twenty-five 100km squares which are identified by a letter, again A - Z omitting I ( the second letter quoted in a reference).
These squares are divided into smaller squares by grid lines representing 10 km spacing each numbered 0 - 9, from the south-west corner in an easterly and northerly direction. You can thus identify a 10km grid square by quoting two grid letters and the eastings and northings; for example, TQ 6 3.
On OS Landranger Maps, you will find the two grid letters on the legend or the corner of the map. The 10km grid is then further broken down into 1km grid squares.
By estimating the eastings and northings to one tenth of the grid interval, you can quote a full six figure grid reference that is accurate to 100m on the ground. For example, the Tower of London's grid reference is TQ 336805.
Geographical origin of the National Grid
The National Grid applies to all Ordnance Survey maps at all scales. Northern Ireland and the Republic of Ireland use their own National Grid system. It covers the whole of Great Britain and extends 700 km to the east and 1300 km to the north of the false origin. It is based on the Transverse Mercator Projection (explanation).
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 (that is, to the east and north of origin 00) 400km are added to all eastings coordinates and 100km 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 1000km for some points in northern Scotland.
History of the National Grid
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 through out World War II. In the military grid, areas were broken down into progressively smaller squares, with sides in turn representing 500km, 100km, 10km, 1km. Letters were allocated to the 500km and 100km squares and numbers to the 10km 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 5000 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 Ordnance Survey 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.