A theodolite is a graduated horizontal circle with a sighting arm — later a telescopic sight and a vertical circle too — that measures angles precisely from a levelled tripod. The word first appears in Leonard and Thomas Digges's A Geometrical Practise, named Pantometria (1571), which describes a set of surveying instruments; Gemma Frisius had set out triangulation, fixing points by intersecting sightlines from a measured baseline, in 1533. From the mid-18th century robust optical theodolites allowed whole countries to be triangulated: the Cassini survey of France (1744–1793), the Ordnance Survey, and the Great Trigonometrical Survey of India. The output was the accurate topographic sheet on which plans could be drawn to scale.
Accurate angular measurement made the metric base map possible, and with it property cadastres, contour drawing, railway and canal alignment, and any planning claim that could be checked. Planning as a drawing-based discipline depends on this instrument chain: no reliable survey, no scaled plan, no zoning boundary. National triangulation also made territory legible for taxation, conscription and colonial administration.
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