Ordnance Survey coordinate systems

Let’s now take a look at the coordinate systems used by OS for mapping Great Britain. There are three coordinate systems to consider:

• OS Net, a modern 3-D TRF using the ETRS89 datum (as described in 'European Terrestrial Reference System 1989 (ETRS89)' ). This coordinate system is the basis of modern Ordnance Survey ‘control survey’ (the surveyor’s jargon for adding local points to a TRF for mapping purposes), and is the basis of definition of all Ordnance Survey coordinates. A subset of OS Net (see section 5.1) has been ratified as the official densification of ETRF89 in GB.

• The National Grid, a ‘traditional’ horizontal coordinate system, which consists of: a traditional geodetic datum (see 'We need a datum') using the Airy 1830 ellipsoid; a TRF called OSGB36 (Ordnance Survey Great Britain 1936) which was observed by theodolite triangulation of trig pillars; and a Transverse Mercator map projection (see 'Transverse Mercator map projections') allowing the use of easting and northing coordinates. This coordinate system is important because it is used to describe the horizontal positions of features on British maps. However, its historical origins and observation methods are not of interest to most users and will be skipped over in this booklet. National Grid coordinates are nowadays determined by GNSS plus a transformation rather than theodolite triangulation.

• Ordnance Datum Newlyn (ODN), a ‘traditional’ vertical coordinate system, consisting of a tide‑gauge datum with initial point at Newlyn (Cornwall), and a TRF observed by spirit levelling between 200 fundamental bench marks (FBMs) across Britain. The TRF is densified by more than half a million lower-accuracy bench marks. Each bench mark has an orthometric height only (not ellipsoid height or accurate horizontal position). This coordinate system is important because it is used to describe vertical positions of features on British maps (for example, spot heights and contours) in terms of height above ‘mean sea level’. Again, its historical origins and observation methods are not of interest to most users. The word ‘Datum’ in the title refers, strictly speaking, to the tide‑gauge initial point only, not to the national TRF of levelled bench marks.

Because triangulation networks need hilltop stations whereas levelling networks need low‑lying, easily accessible routes, there are hardly any common points between the height bench mark network and the OSGB36 horizontal network. OS Net provides a single 3-D TRF that unifies ODN and OSGB36 via transformation software (see 'Geodetic transformations'). Using transformation techniques, precise positions can be determined by GNSS in ETRS89 using OS Net and then converted to National Grid and ODN coordinates. This is the approach used today by Ordnance Survey.