Benchmark

"Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling."


Of more than half a million benchmarks at one time, some may have been lost and the remaining ones are not always easy to spot, but this benchmark is more distinctive than many in that it incorporates a bolt.