Category:Benchmarks in the United Kingdom

English: The vast majority of benchmarks in the United Kingdom have been placed by the Ordnance Survey. Bench marks are the visible remains of height measurements made by the Ordnance Survey. When used in conjunction with older Ordnance Survey maps they indicate the height above sea level at the point of the bench mark. The maps show a bench mark symbol, the letters BM (or B.M.) and the measured height above sea level. All heights are measured relative to the sea level measured at Newlyn in Cornwall. This is known as Ordnance Datum Newlyn, or more commonly ODN, which is the mean level of the sea at Newlyn, Cornwall. This was established using an automatic tide gauge, with measurements collected from the south pier between 1915 and 1921.