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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.

Bench marks Link were historically used to record the height above sea level of a location as surveyed against the Mean Sea Level data (taken at Clarendon Dock, Belfast, for Northern Ireland data, Newlyn in Cornwall for data in Great Britain and Portmoor Pier, Malin Head, for data relating to the Republic of Ireland). They were used as part of a greater surveying network by the UK Ordnance Survey, Ordnance Survey Northern Ireland (OSNI) and the Ordnance Survey Ireland (OSI). If the exact height of one bench mark is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. In this way hundreds of thousands of bench marks were sited all around the UK & Ireland from the mid 19th to late 20th centuries. There are several distinct types of bench mark:

- Fundamental bench marks have been constructed at selected sites where foundations can be set on stable strata such as bedrock. Each FBM consists of a buried chamber with a brass bolt set in the top of a granite pillar. See NG8825 : Dornie fundamental bench mark for an example. FBMs were used in Ireland as well as GB but those in Ireland do not have any surface markers, nor are they marked on standard maps.
- Flush brackets consist of metal plates about 90 mm wide and 175 mm long. Each bracket has a unique serial number. They are most commonly found on most Triangulation Pillars, some churches or on other important civic buildings. See J3270 : Flush Bracket, Belfast for an example.
- Cut bench marks are the commonest form of mark. They consist of a horizontal bar cut into a wall or brickwork and are found just about anywhere. A broad arrow is cut immediately below the centre of the horizontal bar. See J3372 : Bench Mark, Belfast for an example. The horizontal mark may be replaced by or contain a bolt - see J1486 : Bench Mark, Antrim.
Other marks include:
- Projecting bench marks such as SD8072 : Projecting Bracket Benchmark on St Oswald's Tower
- Bolt bench marks such as SJ1888 : OSBM bolt on Hilbre Island
- Rivet bench marks such as J3978 : Bench Mark, Holywood
- Pivot bench marks such as SJ2661 : Pivot bench mark on Leeswood Bridge

Bench marks are commonly found on older buildings or other semi-permanent features such as stone bridges or walls. Due to updated mapping techniques and technological advances such as GPS, bench marks are no longer maintained. Many are still in existence and the markers will probably remain until they are eventually destroyed by redevelopment or erosion.

The re-triangulation of Northern Ireland by the Ordnance Survey Northern Ireland began in 1950. This was the first complete survey of Northern Ireland which included observations with the new primary triangulation of the country, its connection with the Republic of Ireland and the cross-channel connection between Ireland and Great Britain. This began by OSNI establishing a series of triangulation stations throughout the country. Almost all of these stations were topped by trig pillars and 80, mainly primary and secondary pillars, had been constructed by October 1949. Measurements between primary stations began in 1950 and measurements for these and the secondary stations were completed by July 1956. The construction and measurements for tertiary stations were completed later (probably no later than the mid 1960s). Only two stations are not topped by pillars - Lighthouse Island, marked by a brass rivet, and Ardglass, which utilised the top of a high stone folly. The older triangulation stations on the Lough Foyle Base Line were also re-surveyed as part of this process. A majority of the stations are still extant today but a few have been removed or destroyed.