10 Aug 2026 OS Base Line at Magilligan – Dr Finnian Ó Cionnaigh
Construction of the Base Line on Magilligan Strand for the Ordnance Survey of Ireland in the early 19th century
Dr. Finnian Ó Cionnaigh
The Magilligan baseline is a precise 41,640,9 foot (12,692 m) straight line measured in 1828 along the Magilligan strand and Ballykelly in Co London/Derry. Chosen for its flatness, it served as the foundational starting point for the first major Ordnance Survey of Ireland’s triangulation network.
The measurement achieved extraordinary accuracy; when the line was remeasured using electronic equipment in 1960, the difference was roughly one inch (an error of 2 millionths of the total length)
