The grid is based on the OSGB36
datum (Ordnance Survey Great Britain 1936, based on the
ellipsoid), and was introduced after the
retriangulation of 1936–1962. It replaced the previously used
Cassini Grid which, up to the end of
World War Two, had been issued only to the military.
The Airy ellipsoid is a regional best fit for Britain; more modern mapping tends to use the GRS80 ellipsoid used by the
Global Positioning System (the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the GRS80 ellipsoid, and to be slightly less flattened). The British maps adopt a
Transverse Mercator projection with an origin (the "true" origin) at
2° W (an offshore point in the
English Channel which lies between the island of
Jersey and the French port of
 Over the Airy ellipsoid a straight line grid, the National Grid, is placed with a new false origin to eliminate negative numbers, creating a 700 km by 1300 km grid. This false origin is located south-west of the Isles of Scilly. The distortion created between the OS grid and the projection is countered by a scale factor in the longitude to create two lines of longitude with zero distortion rather than one. Grid north and true north are only aligned on the 400 km easting of the grid which is 2° W (OSGB36) and approx. 2° 0′ 5″ W (
OSGB 36 was also used by
nautical charts until 2000 after which
WGS 84 has been used.
geodetic transformation between OSGB 36 and other terrestrial reference systems (like
WGS 84) can become quite tedious if attempted manually. The most common transformation is called the
Helmert datum transformation, which results in a typical 7 m error from true. The definitive transformation from ETRS89 that is published by the OSGB is called the National Grid Transformation OSTN02.
 This models the detailed distortions in the 1936–1962 retriangulation, and achieves backwards compatibility in grid coordinates to sub-metre accuracy.
Datum shift between OSGB 36 and WGS 84
The difference between the coordinates on different datums varies from place to place. The
latitude positions on OSGB 36 are the same as for
WGS 84 at a point in the Atlantic Ocean well to the west of Great Britain. In
Cornwall, the WGS 84 longitude lines are about 70 metres east of their OSGB 36 equivalents, this value rising gradually to about 120 m east on the east coast of
East Anglia. The WGS 84 latitude lines are about 70 m south of the OSGB 36 lines in South
Cornwall, the difference diminishing to zero in the
Scottish Borders, and then increasing to about 50 m north on the north coast of
Scotland. (If the lines are further east, then the longitude value of any given point is further west. Similarly, if the lines are further south, the values will give the point a more northerly latitude.) The smallest datum shift is on the west coast of Scotland and the greatest in
Datum shift between OSGB 36 and ED 50
These two datums are not both in general use in any one place, but for a point in the
English Channel halfway between
ED50 longitude lines are about 20 m east of the OSGB36 equivalents, and the ED50 latitude lines are about 150 m south of the OSGB36 ones.
Illustration of the Ordnance Survey National Grid coordinate system, with
as an example