This data, which is no longer maintained, is available for use under Open Government Licence terms.
Most commonly, benchmarks (BMs) are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
Benchmarks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental benchmarks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN.
On some of the offshore islands there are local mean sea level datums. These have no link in to ODN and must be treated the same way as the mainland levelling.
Global Positioning System (GPS) and the OSGM15 TM model (the height transformation between the European Terrestrial Reference System 1989 and the national height datums) is the preferred method of heighting used by Ordnance Survey. This method achieves the most accurate and uniform method of heighting.
There are approximately 500 000 'lower order' BMs still remaining. This number is reducing due to property development, road widening and so on. The BM heights shown have not been maintained for 30 years and should not be relied upon to accurately define ODN.
There are approximately 190 FBMs, which as the name suggests are our high-accuracy BMs. These are still maintained and are still used by Ordnance Survey. They form our primary height network and, as such, are our link to the Ordnance Datum at Newlyn. They are used in the creation of the orthometric to GPS height correction model (OSGM15). The FBMs do not currently appear in search results but are included in the archive CSV file to download.