Shropshire History

Shropshire

Bench Marks

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https://upload.wikimedia.org/wikipedia/commons/a/a5/Benchmark_in_Edinburgh.jpg

Typical Benchmark (Ordnance Survey)

Bench Marks are survey marks made by the Ordnance Survey to record the height at a given spot in the UK above Ordnance Datum (mean sea level at Newlyn in Cornwall). If the exact height of one is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. They were used when surveying maps in the area.

 

Branching out from Newlyn, a network about 190 Fundamental Bench Marks was created in the UK. These were about 25 miles apart and had very stable foundations to minimise any movement. Each has a buried chamber with a brass bolt set in the top of a granite pillar. They are highly accurate height stations still used today as the baseline to levelling.

 

From these, around 500,000 ordinary bench marks were created, mostly on buildings or other semi-permanent structures. They consist of a chiselled horizontal mark, into which an angle-iron could be placed to form a "bench" for a levelling rod. This ensured that a levelling rod could be accurately repositioned in the same place in the future. The marks were usually indicated with a chiselled arrow below the horizontal line. Sometimes the bench mark is on a bronze or aluminium disk, set in stone or concrete, or on rods driven deeply into the earth to provide a stable elevation point. If an elevation is marked on a map, but there is no physical mark on the ground, it is called a spot height.

 

The first levelling project across England, Wales and Scotland, was carried out between 1841-60.  The first datum was an arbitrary height 100ft 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 In 1844 a datum at Liverpool superseded this. A second levelling project using 115 fundamental bench marks took place between 1912-21. A third levelling project using all of the fundamental bench marks was carried out between 1951-56.

 

It was policy to maintain about 5 bench marks per 1 km square in rural areas and about 30-40 in urban areas. They were regularly checked and renewed to compensate for losses due to building and road works. There was a particular problem in mining areas where subsidence affected the height. Whereas the Fundamental Bench Marks are still maintained, all the others have not been for the last 30 years so they are gradually disappearing. It is estimated that around half have already gone. Like trig points, the original function of bench marks has now been overtaken by use of Global Positioning Satellites.

 

 

 

A detailed list of bench marks in Shropshire can be

found on the website  Bench Marks Database

 

Click on the appropriate map to see the details eg