Carved quietly into the stone of buildings, bridges, and churches across Britain, these small symbols once formed the backbone of the nation’s mapping system. Each mark, a horizontal line above a small arrow, was made by the Ordnance Survey during the nineteenth and early twentieth centuries. The line recorded an exact height above sea level, measured from a fixed point in Cornwall known as Ordnance Datum Newlyn. Thousands of these benchmarks were created as surveyors mapped the country with rem