Benchmarks

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Summary

‘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. Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.’ (Benchmark locator website)

Benchmarks are marked by an arrow on the OS 25-inch maps (1878 1st edition. Numerous marks are shown on the maps, but few survive. A thorough survey of the two islands would be good.

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