Geodesic vertex

A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon, or trig 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 or triangulation stations in North America, trig points in the United Kingdom, trig stations or points in Australia and New Zealand, and trig beacons in South Africa; triangulation pillar is the more formal term for the concrete columns found in the UK.

The station is usually set up by a government with known coordinate 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.

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; however, the 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.

A national geodetic survey and adjustment carried out in the early 1970s in Australia has left a legacy of trig stations consisting of a ground mark with a white quadripod supporting a black disc above the ground mark. These trig stations are clearly visible for many kilometres and useful for hikers.

...
Wikipedia