National Grid Transformation OSTN15 (ETRS89–OSGB36)

To cope with the distortions in the OSGB36 TRF, different transformations are needed in different parts of the country. For this reason, the national standard datum transformation between OSGB36 and ETRS89 is not a simple Helmert datum transformation. Instead, Ordnance Survey has developed a ‘rubber-sheet’ style transformation that works with a transformation grid expressed in easting and northing coordinates. The grids of northing and easting shifts between ETRS89 and OSGB36 cover Britain at a resolution of one kilometre. From these grids, a northing and easting shift for each point to be transformed is obtained by a bilinear interpolation. This is called the National Grid Transformation OSTN15, and it is freely available, as an online service, a software package or as raw data for developers, from the Ordnance Survey website.

The National Grid Transformation copes not only with the change of datum between the two coordinate systems, but also with the TRF distortions in the OSGB36 triangulation network, which make a simple datum transformation of the Helmert type limited to applications requiring an accuracy level of only about 3 metres. OSTN15 removes the need to estimate local Helmert transformations between ETRS89 and OSGB36 for particular locations.

Because the National Grid Transformation works with easting and northing coordinates, other ETRS89 coordinate types (3D Cartesian or latitude and longitude) must first be converted to eastings and northings. This is done using the same map projection as is used for the National Grid (see 'Transverse Mercator map projections'), except that the GRS80 ellipsoid rather than the Airy 1830 ellipsoid is used. The parameters and formulae required to obtain these ETRS89 eastings and northings are given in 'Datum, ellipsoid and projection information' ,'Converting between 3D Cartesian and ellipsoidal latitude, longitude and height coordinates', and 'Converting between grid eastings and northings and ellipsoidal latitude and longitude'. After the transformation, the resulting National Grid eastings and northings can be converted back to latitude and longitude (this time using the Airy ellipsoid) if required.

OSTN15 is the definitive OSGB36/ETRS89 transformation. OSTN15 in combination with the ETRS89 coordinates of the OS Net stations, rather than the old triangulation network, define the National Grid. This means that, for example, the National Grid coordinates of an existing OSGB36 point, refixed using GNSS from OS Net and OSTN15, will be the correct ones. The original archived OSGB36 National Grid coordinates of the point (if different) will be wrong, by definition, but the two coordinates (new and archived) will agree on average to better than 0.1 m (0.1 m rmse, 68% probability).