The Board of Ordnance three-foot theodolite, 1858.
© Science Museum / Science & Society Picture Library
This engraving of Jesse Ramsden's three foot theodolite was published in 1858. The theodolite itself is held in the Science Museum, London. It is the oldest surviving theodolite of its size and was used on the Principal Triangulation of Great Britain and Ireland from 1792 to the early 19th century. The theodolite was used to measure both vertical and horizontal angles. This produced the network of reference points underpinning Ordnance Survey maps.