Triangulation station

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.

To aid the mapping of the country, the science of trigonometic surveying was introduced by Major Thomas Mitchell who had been brought out to the colony as Assistant Surveyor General of New South Wales.

[5] 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).

[6] These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.

They were erected by the Instituto Geográfico Nacional, usually painted in white, and can be marked with a metallic label with the warning: "The destruction of this sign is punishable by law."

On the top a brass plate with three arms and a central depression is fixed, known as a "spider": it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points.

The standard trig point design is credited to Brigadier Martin Hotine (1898–1968), head of the Trigonometrical and Levelling Division of the Ordnance Survey.

"[9] From the same source: "Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the entire length of Great Britain."

[10] The image is the spire of the Fair Haven, Vermont First Baptist Church, a horizontal survey mark (triangulation station), and is described in the NGS National Spatial Reference System under the permanent ID OD1373.