Concept
A triangulation station, also known as a trigonometrical point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity. The nomenclature varies regionally: they are generally known as trigonometrical stations or triangulation stations in North America, trig points in the United Kingdom, trig pillars in Ireland, trig stations or trig points in Australia and New Zealand, and trig beacons in South Africa. The station is usually set up by a government with known coordinates and elevation published. Many stations are located on hilltops for the purposes of visibility. A graven metal plate on the top of a pillar may provide a mounting point for a theodolite or reflector, often using some form of kinematic coupling to ensure reproducible positioning. Trigonometrical stations are grouped together to form a network of triangulation. Positions of all land boundaries, roads, railways, bridges and other infrastructure can be accurately located by the network, a task that is essential to the construction of modern infrastructure. Apart from the known stations set up by government, some temporary trigonometrical stations are set up near construction sites for monitoring the precision and progress of construction. Some trigonometrical stations use the Global Positioning System for convenience. Its accuracy depends on factors such as ionospheric and tropospheric propagation delay errors. Although stations are no longer required for many surveying purposes, they remain useful to hikers as navigational aids. Trig points or Triangulation Pillars are another way of spotting the top of a mountain on a map. The symbol for a trig point is a small triangle. They are real concrete pillars that are placed at particular places which are usually the tops of hills or mountains. A national geodetic survey and adjustment carried out in the early 1970s in Australia has left a legacy of trig stations, many consisting of a ground mark with a black quadripod (pyramid frame) supporting a visible disc above the ground mark.
