Plate of the elevation towers of the Great Arc of India

Plate 1 from Volume II of the Great trigonometrical survey of India showing the elevation towers built across Indian under the direction of George Everest FRS to conduct the survey of India for the East India Company.
The towers were required on flat parts of the land to triangulate the measures. The plate shows diagrams of the exterior and interior of the towers which were built across the Indian subcontinent with many still standing.
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.