Modern surveying and positioning are, invariably, via GNSS methods. The underlying coordinate reference frame for GNSS positioning in GB is the ETRS89 (European Terrestrial Reference System 1989). OS mapping uses a different (so called “legacy”) system – OSGB36 British National Grid and heights are related to mean sea level based datums such as ODN (Ordnance Datum Newlyn). A coordinate transformation is required to link these different systems.
OSTN15 and OSGM15 are the transformation models that link 3D positions measured using GNSS with the national coordinate and height reference systems of Great Britain. OSTN15 models the relation to/from OSGB36 British National Grid and OSGM15 models the relationship to/from mean sea level-based heights.
