Plate of the theodolite constructed for the East India Company

Plate 3 from Volume II of the Great trigonometrical survey of India showing the 3-feet theodolite built by the London-based instrument makers, Troughton and Simms for the East India Company to conduct the survey of India.
These very large optical instruments were used for measuring angles to survey the land in the early years of the Great trigonometrical survey.
The Great Trigonometrical Survey of India was part of the Survey of India initiated by the East India Company to survey the Indian sub-continent during its rule over the territory. The survey recorded topographical information, sounded depths of the navigable waters and recorded land-ownership across the region, but alongside, it set out a complex network to take trigonometrical measures and compute the distances and heights.
Over 70 years, between 1802 and 1871, the survey was conducted with scientific precision and resulted in a variety of scientific advances, from calculating the height of the Himalayan summits to improving the measurement of the curvature of the Earth.