Ramsden's theodolite was used for measuring the angles in the Primary Triangulation of Great Britain. This was the first accurate survey of Britain and formed the foundation for the Ordnance Survey maps of the country.

The huge project was initiated by General William Roy, who had advocated a national survey since 1763. The government at first declined, on grounds of cost, but the impetus to start finally came from a French request to the Royal Society to collaborate and measure the relative positions of the London and Paris observatories. The enterprise appealed to the scientific spirit of George III who helped to fund the work. This initial project subsequently grew into the national Ordnance Survey.

Jesse Ramsden was one of the foremost instrument-makers of his age. Two Ramsden theodolites were used in the survey and the first could take a bearing on a mark 70 miles away with an error of only 1/180th of a degree. The theodolite shown here is the second instrument, which was still more accurate. Despite their great weight, the theodolites were carried to the tops of mountains, steeples and specially-built scaffolds to obtain lines of sight. The measurements taken during the survey were so accurate that they were used for the next 150 years.

Inv. 1876-1203