Ramsden's three foot geodetic theodolite, 1792.
© Science Museum / Science & Society Picture Library
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.