This theodolite, the oldest surviving of its size, was used on the 'Principal Triangulation of Great Britain and Ireland' from 1792 to the early 19th century. It was able 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.
Credit: Ordnance Survey
Object no: 1876-1203