Information on the National Grid

The National Grid provides a unique reference system based on the Transverse Mercator Projection , which can be applied to all Ordnance Survey maps of Great Britain at all scales.

History

Not until World War I was the problem of creating a national reference system seriously addressed, although the departmental committee of 1881 made a thorough investigation of the question of indexing maps at various scales.

For military purposes, a workable grid by which points could be located and referenced was essential and, out of this need, developed the so called 'British Grid System'. This was adopted on military maps in 1919 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 and was overprinted on the War Office editions of Ordnance Survey maps. It should not be confused with the present National Grid.

The development of the National Grid

The National Grid, although influenced by the War Office Grid, was also affected by Ordnance Survey experiments on its regular series of maps.  A grid, for example, with lines 5000 yards apart and giving full co ordinates, was authorised in 1929 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.

Using the National Grid

The National Grid, like its military predecessors, consists of a systematic breakdown of the Grid area 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).
On Ordnance Survey maps 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. So a 10km grid square can be identified using the two letters i.e. TQ and then the eastings and northings i.e. TQ 6 3.

On Landranger Maps the two grid letters i.e. TQ or SU etc can be found 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 a full six figure grid reference can be specified which is accurate to 100m on the ground.

The Origin of the National Grid

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 (i.e. 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.

Source: J B Harley, Ordnance Survey Maps , A Descriptive Manual. Southampton OS 1975. pg 24.

Common Abbreviation: The abbreviation 'NG' stands for National Grid.

Source: Module 6, Section L, Revised to Amendments Nos 2 (May1986) and 4 (October 1987).

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