The grid is based on the
OSGB36 datum (Ordnance Survey Great Britain 1936). The
datum was introduced after the
retriangulation of 1936–1962. It replaced the
Cassini Grid which had previously been the standard projection for Ordnance Survey maps. A
datum transformation exists between GRS36 and more recent geocentric frames (see below). OSGB36 is based on the
Airy ellipsoid, a
reference ellipsoid named after
George Biddell Airy. Introduced in 1830, it is a best fit for the Britain region. More modern mapping tends to use the
GRS80 or
WGS 84 ellipsoid, as used by the
Global Positioning System. The Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the global/world ellipsoid, and to be slightly less flattened. The British maps adopt a
transverse Mercator projection with an origin (the "true" origin) at
49° N,
2° W (an offshore point in the
English Channel which lies between the island of
Jersey and the French port of
St. Malo). 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. In order to minimize the overall scale error, a factor of 2499/2500 is applied. This creates two secant lines of longitude about 180 km east and west of the central meridian along which the local scale factor equals 1, i.e. map scale is correct. Inside these lines the local scale factor is less than 1, with a minimum of 0.04% too small at the central meridian. Outside these lines the local scale factor is greater than 1, and is about 0.04% too large near the east and west coasts. Grid north and true north are only aligned on the central meridian (400 km easting) of the grid which is 2° W (OSGB36) and approx. (
WGS 84). A
geodetic transformation between OSGB36 and other terrestrial reference systems (like
ITRF2000,
ETRS89, or
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. The definitive transformation from ETRS89 that is published by the Ordnance Survey is called the National Grid Transformation OSTN15. 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
longitude and
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 meridians 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 parallels 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. The smallest datum shift is on the west coast of Scotland and the greatest in
Kent.
Datum shift between OSGB 36 and ED50 These two datums are not both in general use in any one place, but for a point in the
English Channel halfway between
Dover and
Calais, the
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. == Summary parameters of the coordinate system ==