Triangulation station

A triangulation station is a fixed surveying station, used in surveying. 
It may also be known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point, and sometimes informally as a trig,
The names of triangulation stations vary regionally; they are generally known as "trigonometrical stations" in North America, "trig points" in the United Kingdom, New Zealand and Australia, and "trig beacons" in South Africa; "triangulation pillar" is the more formal term for the concrete columns found in the UK.