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Between 1783 and 1853 the Ordnance Survey carried out a first-order triangulation of Great Britain and Ireland. The scale of this Principal Triangulation rested on two bases, one on the eastern shore of Lough Foyle and the other in Salisbury Plain. Both were measured by Major General Thomas Frederick Colby, the former in 1827-28 and the latter in 1849.
The Lough Foyle Base Line was a measured line through three fixed triangulation stations. Each station was solidly constructed and consisted of heavy blocks of Dungiven sandstone on which rested the centre point of the station, made with a needle on platina wire set in lead and concrete; this was then covered with a heavy protective flag-stone on which another mark was made directly above the platina wire (this is the Bench Mark). Measuring began on the 6th September 1827 and continued until the 20th November in the following year, although the actual measurements only took some 60 days. The survey was carried out with tools especially developed for the project, most notably an iron and brass compensation bar, a strong Limelight and a heliostat reflector for daylight observations, all developed by Lt Thomas Drummond, a leading mathematician and inventor. The length of the base line was calculated to be 41,640.8873 feet, or roughly eight miles. After the initial survey was complete the stations were covered with earth and left alone.
In 1960 the stations were uncovered for the re-triangulation of Northern Ireland and they were found to be in perfect states of preservation. Measured with modern equipment the original calculations by Major Colby were found to be extremely accurate; the 1960 measurement was greater than the 1827 value by approximately one inch and this represented an agreement of better than one part in half a million.
Three of the original stations remain today:

C6322 : South Base Station, Lough Foyle Base Line located at C6347222158
C6530 : Minearny Base Station, Lough Foyle Base Line located at C6594730597
C6734 : North Base Station, Lough Foyle Base Line located at C6704534340

A triangulation station, Mount Sandy, which was an extension of the original base line and located at C6803037710, has sadly been lost to the sea.

Each remaining station has been protected with a stone wall and high spiked fence; the apparatus inside remains untouched but has been covered by a mound of earth.