Great Trigonometrical Survey of India begins

Background. William Lambton, an English soldier and surveyor, proposed a triangulation survey to establish the shape of the Earth and map the peninsula between Madras and Mangalore. Although a baseline was attempted in Bangalore in 1800, it was rejected for lack of connection to triangulations. The official start occurred in 1802 near Madras, with careful survey equipment having arrived in 1801.

Methodology. The initial baseline was measured with extreme care because subsequent survey accuracy depended on it. Lambton employed special precautions to account for chain expansion due to Indian weather. He used a Great Theodolite made by William Cary for primary triangles and repeated measurements with the Zenith sector to correct for collimation errors and refraction, preferring stars like Aldebaran for latitude determination.

Initial Execution. The baseline measurement began on 10 April 1802 near Madras. It was completed by 22 May 1802, covering 7.5 miles. This baseline served as the foundation for triangulating across the peninsula. The survey was initially focused on the newly acquired territory of Mysore, with the goal of extending across the entire subcontinent, though the name 'Great Trigonometrical Survey' was only officially adopted in 1818.