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Mynydd Cilan Triangulation Station

English tourists gather round the Mynydd Cilan trig at 117m above sea level. Hell's Mouth Beach can be seen in the background.

Trig points were a tool to give surveyors a frame of reference for surveying the UK in the days before electronic positioning aids were available (GPS etc). Typically, a trig point is a concrete post set on a high point such as a hill with a metal disc inset in the top which you can put your theodolite in. The trig point is at an accurately surveyed and documented position so you can survey other stuff relative to one or more trig points.
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