Dartmoor Explorations

A collection of walks, discoveries, insights and pictures of exploring Dartmoor National Park

Fundamental Benchmarks (FBMs)

There are three ‘Fundamental Benchmarks’ (FBMs) within Dartmoor National Park – Two are from the Ordnance Survey (OS) Geodetic Levelling period(s) and one from the GPS era. The two original FBMs are located at Bridford and Yelverton and were used as part of the chain of OS primary levelling lines linking the whole country to the UK Fundamental Benchmark in Newlyn, Cornwall. The third FBM is located in Yelverton and is part of Ordnance Survey’s network of passive GPS stations throughout Great Britain allowing GPS surveyors to precisely position their survey stations for their coordinate systems.

FBMs in *Geodetic Levelling era: There is a UK Fundamental Benchmark at Newlyn, Cornwall where there is a brass bolt which was the reference point for the whole of the United Kingdom, that is, all heights are referenced to this point. The height of the benchmark (bolt) at Newlyn was established over a six year period from 1915 to 1921, when mean sea level was found and this vertical level transferred to the head of the bolt. The bolt head was 4.751 metres above the 1915/1921 mean sea level and is now 4.551 metres above mean sea level as sea levels have risen. There were at one point around 250 FBMs across the UK on specially selected sites where foundations were set on stable strata such as bedrock so that movement of the mark is minimised. They provided a stable network for levelling and there is one approx. every 40km. The FBMs were ‘joined’ in ‘levelling lines’ using flush brackets, OS Bolts and other benchmarks, then ultimately to the UK FBM at Newlyn. On Dartmoor, the main levelling line was between Bridford and Yelverton. NOTE: *Geodetic means accounting for the earth’s curvature.

FBMs in GPS era: From 1989, OS installed and operated a combination of ‘Passive Stations’ and ‘Active Stations’ as part of the Global Positioning System calibration. Some OS ‘Passive’ GPS stations are known as FBM Aux (Auxiliary) and are geodetic quality ground markers (like the previous generation of FBMs) and are in user-accessible locations. (Note: Other ‘Passive Stations used are pillars and bolts from the previous geodetic survey periods). In the UK most of these ‘Passive Stations’ are within 20-35 km of each other. OS have a network of around 1000 ‘Passive Stations’ and around 180 ‘Active Stations” (which are sited at about 30 locations). At the ‘Active Stations’, GPS receivers constantly log readings allowing a very high level of positional accuracy to be achieved, without the need to physically occupy one of the ‘Passive Stations’. The ‘Passive Stations” are surveyed on a five-yearly basis but require the user’s own GPS reference receiver during the survey. On Dartmoor, there is a ‘Passive Station’ (FBM Aux type) at Yelverton, known as Yelverton FBM Aux.

Bridford 1a
This is the granite pillar and gun metal bolt of the Bridford Fundamental Benchmark. These FBMs provide an extremely accurate position and height calibration (computed from the sun and North Star) and were built for endurance. The bolt on the top is the surveyor’s working reference, but the actual datum marker is in a small chamber below the pillar (see cross-section picture later).
Bridford 1b
Sign on the back of the Bridford FBM.
FBM Construction
Fundamental Benchmark construction. Note that there is a chamber next to the granite pillar. (Source: Site Engineering Guide ORDANCE SURVEY: NATIONAL GRID & BENCH MARKS Andrew Burgess B.Sc Hons)
Bridford 1c
The Bridford Fundamental Bench Mark is 843.145 ft above sea level. It is located at SX81190 86507
Bridford 1d
Contextual views of the Bridford Fundamental benchmark. It was used for both the 2nd and 3rd Geodetic Levelling periods 1912-21 and 1950-1968 by Ordnance Survey.
Yelverton 1a
The Fundamental Benchmark at Yelverton is buried under a grass verge alongside the A386, next to where the golf players cross the road.
Yelverton 1b
Using a photograph of the Bridford Fundamental Benchmark, it has been superimposed onto the location of where the Yelverton Fundamental Benchmark should be. It is located at approx SX51639 66461 Further information can be found at: 
Yelverton 1c
Another contextual view of the location where the Yelverton Fundamental Benchmark should be. It was used for both the 2nd and 3rd Geodetic Levelling periods 1912-21 and 1950-1968 by Ordnance Survey.
Yelverton Aux 1a
The Yelverton FBM Aux ‘Passive Station’ is known as a ‘Berntsen’ who is the American manufacturer of the survey equipment. Approximately 100 of the ‘Passive Stations’ installed by the OS, in particular the FBM auxiliaries, used a survey marker from Berntsen. The picture shows cover plate of the Yelverton FBM Aux alongside a cross-sectional view of the survey equipment below ground
Yelverton Aux 1b
Location of the Yelverton FBM Aux ‘Berntsen’. The location is at SX51377 66053, however, measuring from the cattle grid by 7m and from the tarmac by 2m will give a more precise location. Please note the location is near the busy A386, so if visiting this must be taken into consideration. This FBM is not always as exposed as shown in the picture and is often overgrown or covered in a layer of earth.
Yelverton Aux 1c
The lid bears the inscription “SURVEY MONUMENT DO NOT DISTURB” in the centre and “ORDNANCE SURVEY / BENCHMARK” around the outside. The top of the plate cover is not flush with the top surface of soil.
Yelverton Aux 1d
After taking photographs, the author covered up the FBM as it was found

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