Ramsden theodolite

The Ramsden theodolite was used in the first Ordnance Survey of Southern Britain. The theodolite was commissioned from Jesse Ramsden, a leading Yorkshire instrument maker, who had developed the technique of dividing angular scales accurately to within a second of arc. The instrument took three years to build, and had a base circle of 3 ft - it was also known as the Great or 36 inch theodolite.

The full survey (sometimes called the Principal Triangulation of Great Britain) was finally begun in 1791 by a team formed under General William Roy (d 1790); the survey used the new theodolite on a specially surveyed baseline, based Roy's on accurate surveys between London and Paris.

Traces of the theodolite support structure were still to be found many years afterwards at some remote survey points - eg at Soldiers' Lump, the summit of Black Hill (Peak District)

The original theodolite is now in the Science Museum, London.

Ramsden, who was elected to the Royal Society in 1786, also made important contributions to fields such as optics (the Ramsden eyepiece) and electrostatics (the Ramsden machine)