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This theodolite is the oldest surviving of its size and was used on the Principal Triangulation of Great Britain and Ireland from 1792 to the early 19th century. It was used to measure both vertical and horizontal angles to a high degree of accuracy. This helped to produce the network of reference points underpinning the Ordnance Survey maps of the United Kingdom.
The Jesse Ramsden theodolite is a large theodolite which was specially constructed for use in the first Ordnance Survey of Southern Britain in 1787. In 1791 Ramsden's theodolite with a 3 ft. circle reading to 1" built was used for the angle observations and despite its weight of about 300 lbs. good progress was obtained in the triangulation. It is certainly among the 4 or 5 greatest technological advances ever in geodetic surveying. In the field of general surveying however, the invention by the French of the repeating theodolite about 1790 is of equal importance for it is the basis for the instrument most surveyors have employed