Two foot geodetic theodolite made by Troughton & Simms, 1828.
© Science Museum / Science & Society Picture Library
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 conjuction 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.