The Lough Foyle
Base Line
North base line tower in Magilligan/Limavady
The North Base Tower in Magilligan C611344
South base line tower in Ballykelly
The South Base Tower in Ballykelly C634221 


In 1824 The Ordnance Survey of Ireland decided that the lough Foyle shore of our Borough was the ideal place to measure a base line for the map of the rest of the island. They selected this area because it was close to the sea - horizontally as well as vertically - and was close enough to Scotland to tie in with the Hiberno-Scottish triangulation.

Major-General Colby, the leader of the surveyors, designed a special measuring apparatus that compensated for changing temperatures and the project began on the 6th September 1827 and continued until the 20th November in the following year. The length of this line turned out to be 41 640.8873 feet, or roughly eight miles.

The North Base Tower is in a field in Magilligan owned by a rather nice farmer whose name I forgot to ask. The South Base Tower is in Ballykelly at the edge of a football field near the Drummond Hotel. There is also a Minearny Base Tower, about which, more later.

In 1960 the boys from the Ordnance Survey of Northern Ireland came back and re-measured the distance with the latest equipment involving a brand new geodimeter combined with a tellurometer - both instruments representing the cutting edge of 1960 measuring technology. They found a difference of one inch!

Showing the inside of the tower
If, like me, you have always wondered what might be inside these mysterious low towers, don't look any further than this page. Beneath a pile of best Roe Valley top soil is a small chamber, the base of which (B) is formed from blocks of Dungiven sandstone. Set into this foundation is the centre point of the base (A). It consists of a mark made in a platina wire which is set in lead and concrete. Above this is a flagstone (C), which has a metal plate with an edged cross directly above the mark in the wire below.

Remarkable stuff, isn't it?

On top of noble Benevenagh Mountain near the Gortmore viewing point which allows a magnificent view over Lough Foyle, the intrepid traveller can inspect a commemorative plaque, erected by the Society of chartered Surveyors in the Republic of Ireland. (It's the ugly thing behind the the dog's behind).
A commemorative plaque

This is what it says:

The Survey of Ireland
The Lough Foyle Base Measurement

Between the years 1824 and 1846 a major survey of Ireland was carried out by the Ordnance Survey under the direction of Major General Thomas F. Colby. Colby calculated that the most important part of the triangulation for the survey was the determination of scale by means of the precise measurement of a base. In 1824 he selected a site on the flat eastern shore of Lough Foyle for a base which was to be the longest of its kind and measured by methods to a standard of accuracy never before achieved anywhere in the world.

Colby devised an original apparatus for the measurement - a compensation bar of iron and brass about 10 feet long between the pivots, the total length of which was unaffected by temperature changes. Measurement of the base commenced on 6 September 1827, initially under Colby's supervision but later under the direction of Lieutenant Thomas Drummond one of the Ordnance Survey's leading mathematicians. Work was completed on 20 November 1828 having taken the most part of two summers to complete. The length of the base, levelled and reduced to the adjoining sea level, was 41,640.8873 feet or nearly 8 miles. In 1960 the Ordnance Survey of Northern Ireland remeasured the base using electronic equipment; the difference was approximately 1 inch - surely a tribute to the accuracy of the 19th century surveyors.

A base at Salisbury Plain was measured in 1849 using the same apparatus and methods perfected by Colby in Ireland and is connected to the Lough Foyle Base through the principal triangulation network. To preserve the site the Government acquired the land and erected three base towers that can still be seen today. The North Base Tower at Ballymulholland and Minearny Base Tower at Minearny are surrounded by private land and are not readily accessible to the public. South Base Tower, situated at the rear of the Kings Lane Estate in Ballykelly Village, can be visited.

  I have recently learned that there was a fourth tower out towards Lough Foyle and that this was claimed by the sea quite some time ago.

I cannot end this account without showing the Minearny tower dwarfed by noble Benevenagh, the mountain that watched this enterprise when it began and will still be standing when the towers are no more.
Benevenagh with Mineray base line tower

Benevenagh mountain watching over the Minearny tower


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