South Base Tower, Ballykelly, Limavady, Co Londonderry is a Grade B1 listed building in the Causeway Coast and Glens local planning authority area, Northern Ireland. First listed on 22 March 1996.

South Base Tower, Ballykelly, Limavady, Co Londonderry

WRENN ID
hallowed-terrace-quill
Grade
B1
Local Planning Authority
Causeway Coast and Glens
Constituency
East Londonderry MP: Mr Gregory Campbell (DUP)
Country
Northern Ireland
Date first listed
22 March 1996
Address
South Base Tower, Ballykelly, Limavady, Co Londonderry, Mulkeeragh
Irish grid reference
C 634 221
Coordinates
55.04278, -7.00786
Source
NI Environment Agency listing

Also on this page: radon risk · weather exposure · detailed attributes ↓

Description

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.

Local climate

South Base Tower, Ballykelly, Limavady, Co Londonderry sits in a wetter-than-average part of the UK, with an average of 1,140 mm of rain a year. This makes it wetter than 89% of listed buildings in the UK. Its exposure to wind-driven rain is greater than that of 78% of listed buildings.

It experiences an average of 15 freeze-thaw cycles a year, fewer than 85% of listed buildings. Of these, 10 follow at least 1 mm of rain in the preceding 48 hours. Average temperatures at this location have risen by 0.2°C per decade since 1991.

Averages for 1991–2020; temperature trend 1991–2025. Figures describe the wider area (a grid cell of roughly 10 km), not the individual site. Contains modified Copernicus Climate Change Service information 2026.

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