Triangulation pillar on Knocklayd
Saturday 27 January 2024 12:15
COLERAINE Historical Society held their first talk of 2024 on Wednesday January 10) in The Sandel Centre, Coleraine.
Dr. Peter Wilson of Ulster University, gave a talk entitled ‘The Ordnance Survey Legacy in the North of Ireland.’
Peter’s fascinating talk covered the history of the OS in Ireland, whose activities began in 1824, with specific reference to the 'markers' locally across Causeway Coast and Glens.
Kindly summarizing his presentation, for The Chronicle, Dr Wilson, said: 7“The Ordnance Survey was established in Ireland in 1824, under the Directorship of Thomas Colby, with the purpose of surveying the land and preparing detailed maps for use by Government and individuals alike.
“My presentation focused on the early years of the Survey and the physical remains of its work that can be found today in urban and rural locations in the north of Ireland.
“The Principal or Great Triangulation of Ireland was conducted during the years 1824-32. Thirty four hill-top stations were selected and on these summits large cairns with long wooden poles were erected in order to aid identification through theodolite telescopes from distant heights.
“Both Knocklayd and Trostan in Co. Antrim were included in this triangulation scheme. The cairn on Trostan remains to this day.
“Some observations were thwarted by poor visibility but this problem was overcome through use of Thomas Drummond’s ‘limelight’ – an intense light that could be seen at great distances.
“Its value to the Survey was demonstrated in November 1825 when Drummond and a party of Sappers camped on Slieve Snaght in Co. Donegal and directed the limelight towards Divis, the hill that stands above Belfast, during the hours of darkness.
“This experiment demonstrated the value of the method. The remains of Drummond’s camp on Slieve Snaght can be seen today.
“During 1827-28 a baseline of 7.89 miles was measured across Magilligan Foreland in order to aid the triangulation.
“An accurate measurement was needed to facilitate the calculation of distances between the triangulation stations.
“When the baseline was re-measured in 1960, using vastly more sophisticated equipment than was available in the 1820s, the result indicated a difference of just 1 inch – testimony to the method and care adopted in the nineteenth century.
“The Primary Levelling of Ireland took place in 1839-43 and followed parts of the road network. During this process bench marks were inscribed on buildings, roadside walls and bridges – many of these remain to this day and consist of a horizontal line with an upwards pointing arrow below. “Similar features were added at other times as surveying extended into areas not covered during the Primary work.
“A re-triangulation of Northern Ireland was conducted in 1952-56 and used the original stations selected for the triangulation of 1824-32, along with many others.
“Triangulation pillars were constructed on these summits and consist of squat concrete pillars. Each pillar has a small metal plate (a flush bracket) on one side and a metal ‘spider’ on the top, onto which a theodolite can be fixed.
“Today, map making is a digital process and the triangulation pillars and bench marks are redundant. Nevertheless, they played an important role in mapping the nation and deserve to have protected status.”
Dr. Wilson’s highly informative talk was well received, by an appreciate audience, now better informed about the many artifacts, large and small that lie hidden or half hidden in both our rural and urban areas.
One audience member said: “As a hillwalker and hiker, I had absolutely no idea of the origin behind mounds and other structures on top of our main hills such as Knocklayd, Slieve Snaght and Trostan.”
The next talk is on Wednesday February 14, when local journalist and writer, Alan Millar will be giving a talk about John McKinley the Poet of Dunseverick, a now almost forgotten bard, who was all the rage for a few years across Ulster and in Dublin, in the early 1800s.