OSGB36

OSGB36 stands for Ordnance Survey Great Britain 1936, the geodetic datum and associated coordinate reference system historically used by the Ordnance Survey for Great Britain. It provides both a reference ellipsoid and a national grid framework for mapping the mainland of Great Britain.

The datum is based on the Airy 1830 ellipsoid and fixes a reference frame for positions across Great Britain. The corresponding projection is the British National Grid, a Transverse Mercator projection designed to minimize distortion over the region. The grid uses a central meridian of -2 degrees, a scale factor of 0.9996012717 on the central meridian, a false easting of 400,000 metres, and a false northing of -100,000 metres. Eastings increase toward the east and northings toward the north, with coordinates expressed in metres.

OSGB36 has been widely used for Ordnance Survey maps, land titles, and many geospatial datasets covering Great Britain. It does not cover Northern Ireland, which uses other coordinate systems, and is separate from modern global reference frames like WGS84 or ETRS89.

For modern applications, converting OSGB36 data to a global reference frame typically involves a Helmert transformation and a grid-based correction. The preferred approach for high accuracy is to use OSTN02 (or newer) along with the corresponding transformation to convert British National Grid coordinates to WGS84 or ETRS89.

In summary, OSGB36 is the historic geodetic framework underpinning the British National Grid, central to Great Britain surveying and mapping since its adoption in the 1930s.