Two-foot geodetic theodolite with cast iron base and mahogany stand, by Troughton & Simms, London, England, 1826-28. Fitted with three bubble levels and eight reading microscopes. First used in 1828 for checking the Lough Foyle base. With separate parts.

This theodolite was made by Troughton and Simms and first used in 1828 for checking the Lough Foyle base, Northern Ireland. The theodolite was used for measuring both vertical and horizontal angles. This instrument was used for many years in conjunction with the Ramsden three foot geodetic theodolite in the preparation of the Primary Triangulation of Britain, the network of reference points underpinning Ordnance Survey maps.