The first definitive mapping of Great Britain began near Old Sarum and a monument on the A345 commemorates this https://www.stratfordsubcastle.org.uk/mudge-mapping-monument. There are about half a million benchmarks cut by the Ordnance Survey for surveying from 1831 to 1993. From 1921 these were known as Ordnance Datum Newlyn (ODN) points as sea level was taken as a mean at Newlyn, Cornwall. A brass bolt there marks the mean tide level of levels taken hourly from 1915 to 1921 and is the benchmark for the whole of the United Kingdom. However, the Newlyn sea level itself is rising at 1.8 mm a year. These marks are redundant now in the GPS age.

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The horizontal line chiselled out at the top is for an angle iron (L shaped and usually a 90 degree angle) to be placed in it. This gave a platform (a bench) for a levelling rod to be positioned by surveyors. Beneath it is a heraldic broad arrow with a tang between two barbs. It has been used since the 14th Century to denote Royal, Government, Board of Ordnance, Ordnance Survey (as here) and Military (including Navy perhaps as an anchor icon) and MoD use over the centuries. From the 1830’s it appeared on prisoners’ uniforms and still does in any cartoon denoting convicts. In 1875 it became illegal to copy it without authority.