Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of on BM is know then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
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 Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
Bench marks 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 bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for thirty 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 to ODN and must be treated the same way as the mainland levelling.
Global Positioning System (GPS) and the OSGM02TM (the height transformation between the European Terrestria 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 thirty years and should not be relied upon to accurately define ODN.
There 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 CPS height correction model (OSTN02).
The most common type of BM is the cut mark, which is, as the name suggests, cut into a vertical wall, post, standing stone and so on. Others are bolts, rivets, pivots and flush brackets, which are built into most triangulation pillars.
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