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The 'Ramsden theodolite' is a large theodolite (American English: ''transit'') which was specially constructed for use 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 (914 mm) - 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 on Roy's 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, such as at Soldiers' Lump, the summit of Black Hill in the Peak District of England.
The original theodolite is now in the Science Museum in 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).