National Geoid Model OSGM15 (ETRS89-Orthometric height)

A vertical transformation between ETRS89 ellipsoid height and orthometric height is also available from Ordnance Survey. This transformation is based on a gravimetric Geoid model covering Great Britain, Ireland and Northern Ireland aligned with their respective vertical datums. In mainland Britain, OSGM15 therefore gives Ordnance Datum Newlyn (ODN) orthometric heights directly from GNSS survey for direct compatibility with OS mapping, without the surveyor having to visit Ordnance Survey bench marks.

OSGM15 also covers areas of Britain not related to ODN such as the Orkney Islands, Shetland Isles, Outer Hebrides, Scilly Isles and the Isle of Man. Orthometric heights in these areas are related to specific local vertical datums.

In the previous model (OSGM02) all the datums were extended from land to a distance of 10 km offshore and beyond this limit transformation parameters were set to zero. For OSGM15 datums are extended to just 2 km offshore. All areas beyond this are populated with a version of ODN named “ODN Offshore” to signify its lower quality from ODN on land. This approach avoids the arbitrary “cliff” in the transformation data at 10 km but signifies the extended nature (and therefore lower quality) of the datum when used offshore. Away from land “ODN Offshore” values are more closely aligned to the gravimetric geoid rather than the one fitted to the FBMs on land.

The National Geoid Model OSGM15 is a grid of geoid-ellipsoid separation values on the same 1 kilometre resolution grid as the OSTN15 transformation. In the same way as for OSTN15 a geoid-ellipsoid separation value is obtained for any location by bi-linear interpolation. The nominal accuracy of the geoid model is 8 mm rms (root mean square) in mainland UK, better than 2 cm rms for other areas and 3 cm rms for Isle of Man. This transformation is freely available from the OS website. OS recommends the use of the National Geoid Model OSGM15 and OS Net for high-accuracy orthometric heighting, in preference to Ordnance Survey bench marks.