As of 26th August 2016, Ordnance Survey have adopted a new transformation OSTN15/OSGM15 to convert from ETRS89 to OSGB36 co-ordinates. This switch is due to an improvement made by Ordnance Survey in their techniques collecting gravity data. As a result, a new geoid model has been produced for the UK and Ireland, called OSTN15. The geoid is used to link positions recorded using GNSS with national coordinate systems, giving an accurate 3D location within the UK. Further information on OSTN15, the geoid and this update can be found on the OS website. The document “OSGM15 and OSTN15: Updated transformation for UK and Ireland” (M.Greaves et al., Geomatics World, July/August 2016) is particularly useful and can be found here.
To address this new change Plymouth Coastal Observatory have updated the control network around the southwest to OSTN15. The positional change has been minimal, on average 7mm, however, elevation changes of up to 40mm have been observed. As a result, the witness diagrams and shapefile have been updated to the new transformation and can be accessed here.