Travelling around anywhere in Great Britain you will occasionally see a cut bench mark, a horizontal line with a broad arrow beneath it, on a wall, or wherever. The usual reaction is "oh, there's a bench mark, something to do with the Ordnance Survey maps".
Bench marks are marks made by the Ordnance Survey, whose height relative to a declared datum has been determined by spirit levelling. It was policy to maintain about 5 bench marks per 1Km square in rural areas, about 30 to 40 in urban areas, and there was a policy to check and renew marks to compensate for losses due to building and road works. Various sorts of bench marks are used, with varying degrees of accuracy of the height data.
The types of mark are described and illustrated in Harley 1975.
|fundamental bench mark||
Fundamental bench marks are located about 40Km apart
on lines of primary geodetic levelling; based on bedrock
or other stable strata.
In an enclosure of iron railings there is a buried chamber with 2 reference points, a gunmetal bolt and a flint, which are only used for primary levelling. Adjacent to this is a granite or concrete pillar, about 30cm square, with a brass bolt whose height is published.
There are only about 115 (?)in the whole of Great Britain.
It is said that the arrangement looked like a grave, and that one fundamental bench mark outside a church was enhanced by flowers for many years.
|flush bracket||Flush bracket bench marks are placed about 1.5Km apart along lines of geodetic levelling, and at important junctions of lines. They are also placed in the side of most trig point pillars.|
Bridge at Skirwith, NY615359.
The mark is a metal plate, 9x17.5cm, mounted on a vertical wall. The height refers to a small horizontal platform at the point of the broad arrow cast in the metal. Each flush bracket has a serial number.
Projecting bracket bench marks mostly date from the early
stages of the 2nd primary geodetic levelling of 1912-21.
The mark is a metal plate on a vertical wall, with a projecting bracket on which a raised stud is the height reference.
The bolt bench marl is a 6cm diameter brass plate,
engraved with a broad arrow and:-
O S B M
A raised stud in the horizontal plate is the height reference.
|cut bench mark||The cut bench mark is perhaps the most familiar of all bench marks. There is a horizontal line cut in an upright stone surface, with abroad arrow cut below. The centre of the cut line at the point of the arrow is the height reference.|
|Parapet of the bridge over the River Lowther, Bampton Grange, Bampton, NY52071800.|
On a boulder in a drystone wall,
Occasionally there is a metal plug at the reference point, engraved with a horizontal groove.
St James's Church, Temple Sowerby, NY611271; cut bench
mark with an engraved plug, and a spot height:-
348 3/4 feet above the Sea
|rivet||A rivet bench mark is a brass rivet in an approximately level stone surface, with a broad arrow cut to point at the rivet. The top of the rivet is the height reference.|
On a flat rock on the road up Wrynose Pass,
Lakes, NY28370323; hard to see!
|pivot||A pivot bench mark is a small hollow in an approximately level stone surface, with a broad arrow pointing to it. A 5/8 inch ball bearing placed in the hollow is the reference height.|
On a step of a stile, now hidden by plants,
Bench Mark Lists
The Ordnance Survey used to publish bench mark lists, arranged by 1Km square (4 figure grid reference). These lists are now accessible, free, on a website - whose address is so complicated I hesitate to give it here:-
http://benchmarks.ordnancesurvey.co.uk/pls/htmldb/f?p=111:13:18033203823611428938::NO:13::The data is provided in a table which can be downloaded as a .csv (comma separated value) file to load into a spreadsheet program.
The columns are:-
Square, Easting, Northing, Mark type, Description, Height, Order, Datum, Verified year, Levelling year, Metres above groundeg:-
NY,2930,0323,CUT MARK,ROCK S SIDE RD,167.8168,3,N,1956,,0Some other entries are less easy to understand.
Just a reminder:-
1 foot = 0.3048 m
Spirit levelling is the system of finding the altitudes of particular points relative to a know datum. The datum for mainland Great Britain is mean sea level at Newlyn; the Ordnance Datum (OD).
The Leveller had a back stave, vertical pole with height markings, placed on a point of known height, a forward stave, similar, place some distance further on, and a levelling instrument set up in between. The instrument had a telescope with stadia to read off the marks on the staves, kept horizontal by a long spirit level. Observations, carefully horizontal, were made on the back stave and forward stave, and the difference gave the height of the forward position. The forward point became the new back point, and a new forward point was measured, etc. The whole process should finish at another position whose height was known already; and the levelling repeated in the other direction as a check. This is a simplified account! there are problems of curvature of the earth, intervisibility of points, keeping staves vertical, and so on.
Thomas Colby, Superintendent of the Ordnance Survey, was aware of spirit levelling from his involvement in a committee that organised the levelling of a canal route from Axmouth to Portishead, 1837-38. (This canal was never built.)
The first primary levelling across England, Wales and Scotland, was carried out 1841-60. The first datum was an arbitrary height 100 feet below a bench mark cut in the face of the tower of St John's Church, Old Haymarket, London; in 1844 a datum at Liverpool superseded this. Early maps might give bench mark heights relative to the Liverpool Victoria Dock datum, found since to be 0.65 feet below mean sea level.
The primary levelling followed a network of lines across Great Britain, shown on an:-
INDEX MAP of INITIAL LEVELLING in ENGLAND and WALES
The lines in Cumbria are roughly:-
The initial choice of bench marks, excepting the fundamental and important marks, was not always safe, sometimes
on any wall or building that came handy
So later, some had moved, and some had disappeared. Stone walls might be rebuilt, the bench mark moved or lost; milestones are not very stable; and so on.
In local circumstances a surveyor might set up his own bench marks for a project. One fall back is to use a nail in a tree. This has its own problems. A year later, returning to an archaeological site our surveyor found his bench mark nail with dozens of other nails added by local children. While trees grow mostly at the top, a tree even as large as 2 foot diameter will rise a 1/4 to 3/8 inch in a year, in its trunk.
Second Primary Levelling
three new tide gauges were set up, at Felixstowe 1913, Newlyn 1915, and Dunbar 1917. As the resulting mean sea level at the three places did not agree, the Ordnance Survey choose one, Newlyn to be their datum. The tide gauge here was at the end of the harbour pier facing the Atlantic Ocean, and the area was stable granite rock. The Newlyn tide gauge remained an OS responsibility till 1983, when it was taken over by the Institute of Oceanographic Studies. The original tide gauge is preserved at OS headquarters, Southampton.
A second primary levelling, using 115 fundamental bench marks, took place 1912-21. The whole system was more orderly; the flush bracket, projecting bracket and bolt were instituted as intermediate grade bench marks.
Third Primary Levelling, and ...
A third primary levelling was carried out 1951-56.
By the 1970s the systematic maintenance of low grade bench marks was abandoned.
The broad arrow is a mark, or sigillum, of the property of the King or Queen of England. It is said to have been used since the time of the Edwards, when the english bowmen and their arrows were the most powerful military weapon of the time.
The royal use predates the romantic tale that the mark derives from the coat of arms, or a pheon azure, of Henry Sidney, Earl of Romney, Master General of the Ordnance in the late 17th century. A pheon is a similar armorial charge to an arrowhead.
These are references that I happen to have used. There will be dozens of other books about surveying!
Owen, Tim & Pilbeam, Elaine: 1992: Ordnance Survey, Map makers to Great Britain since 1791: Ordnance Survey (Southampton, Hampshire):: ISBN 0 319 00498 8 (pbk)
|private||Individual surveyors, private or commercial, might set up their own local bench marks to suit a job. This mark was found in the road upon a railway bridge in Waverton.|
|And this was found in the path just outside the church in troutbeck.|
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