Ordnance Survey benchmarks: Britain’s mapping legacy
Ordnance Survey benchmarks are among the most significant features of Britain’s historic landscape. Whether carved into stone walls, bridges or buildings, or displayed in a metal plate, these marks formed the backbone of a national system for measuring height. More than 700,000 benchmarks were created across the country from the 1830s to the 1990s, though most are no longer in use. The Ordnance Survey was originally founded in the 18th century to record features and settlements for military purposes. As surveying techniques improved during the 19th century, accurate maps of different scales were developed and published. A consistent vertical reference system was needed. Surveyors had to know not just where places were, but how high they stood above sea level. To achieve this, Ordnance Survey established a network of benchmarks, named for the process of placing an angle iron into a horizontal mark to form a "bench" on which a levelling staff could rest while measuring height. ...