## 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.