Ordnance Survey Benchmarks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
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 fundamental benchmarks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has not been maintained for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values and therefore none of these values should be relied upon to accurately define ODN.
BMs are recorded by their Ordnance Survey map reference location rather than the parish in which they may be found. In order to match the parishes used throughout the "Hearts & Soles" project, I have attempted to utilise the most likely parish for each in these records.
There are approximately 500,000 ‘lower order’ BMs still remaining. This number is reducing due to property development, road widening and so on.
Most commonly, the 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 Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Global Positioning System (GPS) and the OSGM02TM (the height transformation between the European Terrestrial Reference System 1989 and the national height datums) is now the preferred method of heighting used by Ordnance Survey. This method achieves the most accurate and uniform method of heighting.
There are approximately 190 FBMs, which as the name suggests are the high-accuracy BMs. These are still maintained and are still used by Ordnance Survey. They form the primary height network and, as such, are the link to the Ordnance Datum at Newlyn. They are used in the creation of the orthometric to GPS height correction model (OSTN02). The FBMs are not available to the general public.