EPSG:9633

This method is used for transformation between ETRS89 (or WGS 84) geographic and OSGB36 / National Grid projected coordinate reference systems.  It uses a gridded data set with nodes in a temporary projected CRS (ETRS89 / National Grid) for bi-linear interpolation of grid coordinate differences.

The offsets at a point are derived by interpolation within the gridded data. One grid file contains both easting and northing offsets. The grid file format is given in documentation available from the information source. The coordinate reference system for the coordinates of the grid fiule nodes is an intermediary projected CRS, ETRS89 / British National Grid.

For the transformation of ETRS89 coordinates to OSGB36 / British National Grid coordinates, the ETRS89 latitude and longitude are first converted into ETRS89 / National Grid values using the Transverse Mercator formulas given in EPSG method code 9807. Then bi-linear interpolation is used to derive easting and northing offset values. These are added to the temporary ETRS89 / National Grid coordinate values to give OSGB36 / National Grid coordinate values.

For the transformation of OSGB36 / British National Grid coordinates to ETRS89 latitude and longitude, iteration is required. For the first iteration the OSGB36 / British National Grid easting and northing values are assumed to be in the ETRS89 / National Grid. Bi-linear interpolation is used to derive the easting and northing offset values which are applied with sign reversed to the input coordinates.The result is used in further iterations of interpolation and application of offset until the difference between results from successive iterations is insignificant. Finally these Transverse Mercator grid coordinates are converted to ETRS89 latitude and longitude using the formulae given in EPSG method code 9807.