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.
Use
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.(On UK Ordnance Survey maps the symbol is a blue equilateral triangle with a blue dot at the centre) They are real concrete pillars that are placed at particular places which are usually the tops of hills or mountains.
Australia
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.
France
Managed by I.G.N. All 380000 French géodesics terminals are available on https://geodesie.ign.fr/index.php?page=reperes_de_nivellement and on an interactiv & collaborative map tool for smartphone available on google store "géodésie de poche". Each terminal are fully detailled lat/long /altitude RGF93 ellipsoïd location and photo. They can be remarquable points as churches steeple crosses, ground rocks square blocks, or, in town, 12cm cylinders sticked against base walls. Both tools also display GNSS EGNOS permanent base stations.
Hong Kong
Many trigonometrical stations were placed on hilltops around Hong Kong. They strongly resemble those used in other former British colonial territories such as Australia, consisting of a white column topped with a black band.
Japan
In Japan, there are five classes of :
They are installed approximately every , with smaller ones (as necessary) about every . There are about 1000 throughout Japan. The pillars are on a side, and each pillar is anchored with two very large perpendicular rocks buried underground.
They are installed approximately every . There are about 5000 throughout Japan, and the pillars are on a side. Each pillar is anchored with a very large perpendicular rock buried underground.
There are about 32,000 installed throughout Japan, with one approximately every . The pillars are on a side, and each pillar is anchored with a large perpendicular rock buried underground.
They are installed approximately every , and there are about 69,000 throughout Japan. The pillars are on a side, and each pillar is anchored with a large perpendicular rock buried underground.
These markers were installed in 1899 and are the predecessors to the modern triangulation stations used in Japan today. They are generally not used anymore since the installation of the Class 1-4 stations. Some of them still exist at various locations throughout Japan.
New Zealand
there are 5,765 trig stations in New Zealand. They are placed on top of hills and are usually black and white.
South Africa
South Africa has a network of approximately 28,000 trig beacons, established by the Chief Directorate: National Geo-spatial Information (historically known as the Trigonometrical Survey). These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.
