Harston's Bench Marks
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-leveling, 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 leveling 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 leveling framework across the country is provided by 180 Fundamental Bench Marks (FMBs).They are located about 40 km apart along lines of leveling. 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 are no FMBs in Harston.
From these FBMs around 500,000 of lower-order bench marks were established on buildings or other semi-permanent features. The Ordnance Survey has recorded all its benchmarks at:
This table gives details such as height above sea level, map coordinates, type of benchmark, date last inspected and description of location.
With the increasing use of satellite technology and Geographic Information Technology (GIS) since the early 1980s the benchmarking system has become redundant. There has been little maintenance and any bench marks which disappear due to building work, demolition, road widening, erosion, etc, are not replaced. We have seen some of Harston’s benchmarks disappear and other face removal. We will not see them again.
There are two main types of lower order bench marks, both of which can be found in Harston:
a) 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 leveling 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
b) the cut bench mark is by far the most common type which was used from the 1800s to around 30 years ago. They are chiseled into stone, brick or wood on all sorts of vertical structures with a horizontal leveling 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, etc.
The marks which have disappeared from our village were on the bridges over the Rhee on Haslingfield Road and Hoffer Brook on Royston Road which have been rebuilt in recent times. Also no longer in place but retained by our group is the flush bracket which was originally sited on the wall of The Old English Gentleman pub, later Vujon restaurant.
The only remaining flush bracket is on the Trig Point on Rowley’s Hill.
We have discovered the following cut bench marks in the village on:
the church tower,
the old pumping station in Button End,
the wall of The Pemberton Arms,
the garden wall of Park House,
the milestone on London Road,
the railway bridge on London Road,
Enclosure Cottage and Moor Farm barn on Shelford Road
See if you can find them on a wander around the village
There are others recorded on the remains of the old platform platform on Station Road and at the one of the New Farm cottages on Royston Road but we have not seen these.