Ordnance Survey surveyor marks

Lots of discussion on social media this weekend about Ordnance Survey marks on the gritstone of the Dark Peak in the Peak District. A topic that fascinates me with every walk including a foray into the wilderness to try to find some elusive mark made more than a century and a half ago.

Top left is a benchmark used to establish height at a point on the footpath to Stanage End from Moscar.

Below that is a benchmark used to mark a survey position on Higger Tor.

Top right is a really interesting one. An arrow below a square box with a dot in the middle. This denotes a survey height taken from the ground and not estimated. On modern maps today these are denoted as a black dot with a height number as opposed to an orange dot which denotes a measurement taken from the air.

Bottom left is a lovely levelling bolt, indicating a survey position near Laddow Rocks.

Bottom right two survey marks, one for levelling and one for triangulation found on Back Tor on Derwent Edge. On the left hand mark is a benchmark made by Lieut Barlow RE when he carried out the triangulation in 185. The right hand mark denotes a spot height taken in 1854 by Capt Kerr RE to establish the contour lines.

All heights back then used the Liverpool datum and approximate average of the sea height there. Nowadays the OS maps use the Newlyn datum taken from the tidal measuring station in Newlyn Harbour, Cornwall.

 

Ordnance Survey Benchmarks

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Benchmark at Moscar SK 2305 8759

This is a levelling benchmark placed and used by Ordnance Survey. Its location is SK 2305 8759 at Moscar on the footpath from the A57 towards Stanage. The mark was made in 1961, is of the third order of surveying and is 600mm above ground level. Its datum is Newlyn. Originally the mark and boulder were on the west of the footpath but time and boots have now placed it east of the path.

There are some 500,000 benchmarks on the UK mainland, most no longer in use. They identify the height at a given point. The base line is the tidal measuring station at Newlyn in Cornwall, that is the point where all height measurements are taken from, including the 190 Fundamental Benchmarks which were the first order and therefore the most accurate, set in chambers deep underground on bedrock , these benchmarks are still in use today by the Global Positioning System for calculating the accuracy of the height calculation.

SK 24532 94653 Broomhead Moor Fundemental Benchmark TP0526 Chamber
SK 24532 94653 Broomhead Moor Fundamental Benchmark TP0526 Chamber