Ordnance Survey Bench Marks
Bench marks are the visible remains of height measurements made by the Ordnance Survey. When used in conjunction with older Ordnance Survey maps they indicate the height above sea level at the point of the bench mark. The maps show a bench mark symbol, the letters BM (or B.M.) and the measured height above sea level. All heights are measured relative to Newlyn in Cornwall.
The measurements were made and carried around the country by a network of around 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order bench marks were established. There are a small number of standard designs:
- Fundamental Bench Marks (FBM)
- Flush Bracket Bench Marks (FL BR)
- Cut Bench Marks
- Bolt Bench Marks
- Pivot Bench Marks
Fundamental bench marks
Unlike all other types of bench marks, the fundamental bench mark (FBM) is still in use and maintained by the Ordnance Survey. There are 180 FBMs and they provide the basic levelling framework across the country. They are located about 40 km apart along major lines of geodetic levelling. There is a brass bolt at the top of a concrete pillar which is used for everyday purposes, but there are two further reference points (a gunmetal bolt and a flint in a hurried chamber) which are used for principal levelling.
The next most common type of bench mark is the flush bracket. These are common on Ordnance Survey trig points, bridges and church buildings. It is a metal plate about 9cm wide and 17.5cm tall. They are spaced about 1½km apart along all levelling lines and also at important junctions of secondary levelling lines. Each bracket carries a unique reference number.
Cut bench marks
The most common type of bench mark is the cut bench mark which can be found on houses, walls, stones, churches … even boulders. The height is measured at the horizontal bar.
The network has had little maintenance since the early 1980s when satellite technology took over and many bench marks are now “lost” due to building work, demolition and the widening of roads.
The height of the bench marks was established in a process called levelling and measures the heigh relative to Ordnance Datum Newlyn (ODN). There were three such levelling carried out. The first took place between 1840 and 1860 and established the positions of the FBMs. All other bench marks were positioned during the second levelling which was carried out between 1912 and 1921 or the third levelling carried out between 1951 and 1956.
Bench Marks I have Found
You can see a list of the bench marks I have found here.