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More than one visitor to Bishops’ House has asked us what the mark is on the North West corner of the house. One visitor suggested that it could be some sort of mark made centuries ago by a witch. Unfortunately the explanation is much more mundane and anyone who goes out walking will recognise it as an Ordnance Survey trig point. On the 1904 map it is recorded as 485.42 ft above sea level. The 1906 map shows it as 485.4 ft above sea level. Yorkshire was surveyed between 1889 and 1892 and Derbyshire in 1875 for this map. But mapping Yorkshire and Derbyshire started as early as 1808, finally resulting in the First Edition of the Ordnance Survey in 1840. The 1840 OS map does not display spot heights so it is impossible to say when the benchmark was inscribed though it seems a safe assumption to assume that it was during the second half of the nineteenth century.
Source: Hewitt R  Map of a Nation – a biography of the Ordnance Survey. Granta books.
The 1904 1:500 map and the 1906 1:2,500 map, both produced by the Ordnance Survey.