The future of British mapping coordinate systems

OSGB36 and ODN will be retained as the basis of OS mapping for the foreseeable future, but OS Net will be used to access both these coordinate systems by GNSS, using coordinate transformations from ETRS89 to OSGB36 and ODN provided by OS. These transformations are currently the National Grid Transformation OSTN15 and the National Geoid Model OSGM15. In this way, OS surveyors and GNSS‑equipped customers will have access to the national horizontal and vertical datums at any point without visiting traditional control points. Using OSTN15 and OSGM15, OSGB36 and ODN coordinates are obtained by 3‑D transformation software that converts ETRS89 GNSS coordinates obtained from OS Net into the equivalent OSGB36/height datum position.

Using this method of access, OSGB36 and the appropriate datum heights will be continuously accessible, rather than being available only at OS monuments. With the release of OSTN15 (and originally OSTN02), the National Grid has become by definition a transformation of the ETRS89 GNSS coordinate system. Because both horizontal and vertical national mapping coordinate systems are defined in terms of ETRS89, which is easily related to ITRS (see section 6.5 below), the geodetic basis of British mapping is implicitly related to all other national datums that have a known relationship to ITRS.

At some point in the future, the drift of the Eurasian tectonic plate (~25mm/year) relative to WGS84 plus the improved accuracy of stand-alone GPS positioning from even simple “consumer level” receivers will mean the offset between ETRS89 and WGS84 becomes apparent to a much wider group of users, even on small scale mapping. At this time either a change to the epoch of ETRS89 (to closely align it with WGS84) may have to be considered, or the introduction of a small transformation to shift WGS84 coordinates back to ETRS89, before then transforming (e.g. with OSTN15) to OSGB36.

The coordinates and other information of the traditional networks are no longer actively maintained by OS. Access to these traditional control archives is made available through online services on the OS website.