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A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point. Erected by the Ordnance Survey.

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The term benchmark, originates from the chiselled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a levelling rod, thus ensuring that a levelling rod could be accurately repositioned in the same place in future. These marks were usually indicated with a chiselled arrow below the horizontal line.
The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark, a point with a precisely known relationship to the level datum of the area, typically mean sea level. The position and height of each benchmark is shown on large-scale maps.