South Base Tower, Ballykelly, Limavady, Co Londonderry

The South Base Tower, located in the townland of Mulkeeragh near Ballykelly, Co Londonderry, dates to approximately 1825-1839. It marks a key point in the Ordnance Survey’s triangulation of Ireland, undertaken under the direction of Col. Colby. This triangulation was part of a broader effort to create an accurate map of the country.

The tower comprises a low, circular whinstone wall enclosing the original base point, topped with sandstone coping and a metre-high cast iron railing. The wall averages 1.2 metres in height and the circular base has a diameter of roughly 6 metres. This southern base was one end of a line, approximately 8 miles long to Magilligan, used to establish the triangulation pattern. The enclosed area is currently overgrown with weeds.

The building’s significance lies in its industrial archaeological interest and its role as the southern extremity of the Ordnance Survey’s work in the 1830s. The tower is of authorship, historical, and rarity interest relating to Northern Ireland and international context. It was designed by an Ordnance Survey engineer. Further information concerning the first Ordnance Survey is available at the Tower Museum in Londonderry. The listing covers the tower defined by the surrounding wall and railing, and the property is owned by the local government.