- Ramsden theodolite
The Ramsden theodolite is a large
theodolitethat was specially constructed for use in the first Ordnance Surveyof Southern Britain. It was also known as the Great or 36 inch theodolite.
The theodolite was commissioned from
Jesse Ramsden, a leading Yorkshireinstrument maker, who had developed an accurate dividing enginefor graduating angular scales. The instrument was accurate to within a second of arc. The theodolite took three years to build and had a base circle of 3 ft (914 mm).
The full survey, sometimes called the
Principal Triangulation of Great Britain, was begun in 1791by 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 Londonand 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 Districtof England.
The theodolite is now in the Science Museum in London.
Ramsden, who was elected to the
Royal Societyin 1786, also made important contributions to fields such as optics(the Ramsden eyepiece) and electrostatics(the Ramsden machine).
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