How do I relate coordinates from the old and new transformation models?
A. The definitive Ordnance Survey transformation and geoid model OSTN02 / OSGM02 is an improvement over the previous models OSTN97 / OSGM91. The horizontal transformation accuracy has been improved from 0.2m to 0.1m rms and the vertical accuracy has been improved from 0.05m to 0.02m rms. This means that coordinates transformed using both new and old models are very likely to differ. If you have coordinates in OSGB36 which were transformed using the previous transformation (OSTN97 / OSGM91) and wish to relate them to coordinates transformed using OSTN02 / OSGM02 they should first be back-transformed into ETRS89 using OSTN97 / OSGM91 and then forward transformed again into OSGB36 using OSTN02 / OSGM02. An executable is available which will do this back transformation for users who no longer have access to OSTN97 / OSGM91. Download the executable file. Users who have archived their coordinates in ETRS89 will not have to back transform to relate old and new datasets. The archive coordinate
