Spot the benchmarks

John Roadley

Have you managed to spot the carving or cut into the stone at the base of All Saints Parish Church. If you have, you have been looking at a bench mark and many more can be found on walls around Harston, and in fact around the U.K. They are used to give the height above sea level.

The height above sea level of the base of Harston church is 13.041m but where is this sea and how do we know? Since the 1920s, the mean tide level at Newlyn, Cornwall, has been used – the Ordnance Datum. From this point, by surveying and spirit-levelling, the height above sea level for the whole of the UK was established and a series of bench marks constructed by the Ordnance Survey. The horizontal marks are used to support a stable ‘bench’ for a levelling stave to rest on – hence ‘bench marks’. This design ensured that a stave could be accurately repositioned in the future and that all marks were uniform.

Once the exact height of one bench mark is known, the exact height of the next can be found by measuring the difference in heights.

The basic levelling framework across the country is provided by 180 Fundamental Bench Marks (FMBs).They are located about 40 km apart along lines of levelling. There is a brass bolt at the top of a small concrete pillar. Unlike other types of bench marks, the FBM is still in use and they are maintained by the Ordnance Survey. There ae no FMBs in Harston.

From these FBMs around 500,000 of lower-order bench marks were established on buildings or other semi-permanent features. They have had little maintenance since the early 1980s when satellite technology took over. Many bench marks are now “lost” due to building work, demolition, and the widening of roads.

There are two main types of lower order bench marks, both of which can be found in Harston:

The flush bracket (FLBR) commonly found on Survey trig points, bridges and public buildings. It is a metal plate about 9cm wide and 17.5cm tall. They are spaced about 1½km apart along levelling lines. The height is measured at the top of the arrow on the face via a small metal platform that attaches using the two slots at the top. Each bracket carries a unique reference number, the earliest numbered 1 – 3000 date from 1912-21. After this date, the numbers had an ‘S’ prefix. There were 2 of this type in Harston: one on the trig point on Rowleys Hill and the other formerly on The Old English Gentleman pub wall, saved at demolition.

The cut bench mark is by far the most common type which was used from the 1800s to around 30 years ago. They are chiselled into stone, brick or wood on all sorts of vertical structures with a horizontal levelling line with a three line arrow pointing towards it. Each one is unique depending on the mason who cut it; plain, decorated, roughly cut, accurately cut, small, large. There are at least 10 cut bench marks in Harston which can be found along Royston Road, Button End, High Street, London Road and Shelford Road. One is here on the Milestone, London Road and the other above on the church. Can you find the rest?

This page was added on 27/11/2023.

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