Geodetic Transformations Models- the 'Grid Look Up Table' for OSTN02
The OSTN02 (2002) national ‘definitive transformation’ is an accurate realization (conversion) between OSGB36 National Grid and ETRS89 (1989) coordinates.
To achieve the re-projection, a 7 parameter (3 shifts, 3 rotations and a scale change) transformation is required.
- The OSTN02 transformation is modeled as a 1km grid of varying parameters which covers the entire country
- An ETRS89 coordinate (Geodetic Latitude and Longitude or Cartesian XYZ) is first projected to a plane Easting and Northing coordinate using National Grid projection parameters.
- The corresponding 1 km square in the transformation grid is then selected. At each grid intersection there is a set of transformation parameters, expressed as linear shifts between the two systems.
- The four sets of parameters around the chosen grid square are interpolated to give a pair of shifts (one easting and one northing) valid at the ETRS89 coordinate.
- These shifts are applied to the ETRS89 coordinate to transform it to an OSGB36 coordinate. OSTN02 can also perform the reverse operation from OSGB36 to ETRS89.
Source: “The OS’s new Coordinate Transformation for Great Britain – GPS to OSGB36 National Grid Transformation”, by Mark Greaves, Ordnance Survey Geodetic Analyst, and Paul Cruddace, Ordnance Survey Geodetic Advisor, published in the November/December 2001 issue of the ‘Geomatics World’ trade magazine, page 35
Exaggerated view of the OSTN02 transformation grid (each grid cell is 1 km square). The blue arrow indicates the northing and easting shift at a particular point, interpolated from the shifts at the corners of the grid cell in which it falls. In the real transformation the shifts at adjacent grid intersections are almost identical and vary smoothly.