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.
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 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 lines levelling lines and also at important junctions of secondary levelling lines. Each bracket carries a unique reference number.
The network has had little maintenance since the early 1980s and many bench marks are now “lost” due to building work, demolition and the widening of roads.
Of the original million or so bench marks there are approximately 500 000 still remaining.
The height of the bench marks was established in a process called levelling. Lines of fundamental bench marks were established across the country between 1840 and 1860, with a second levelling carried out between 1912 and 1921 and a third levelling carried out between 1951 and 1956. None of the FBMs were changed by the third levelling.
Bench Marks I have Found
You can see a list of the bench marks I have found here.