Great Trigonometrical Survey

From its inception in 1600 to its domination of the entire Indian subcontinent by the beginning of the nineteenth century, the British East India Company gained more and more territory. . This shows surveyors stretching a chain on coffers supported on pickets. The chain is housed under shade to reduce errors due to thermal expansion, and is aligned using a boning telescope. was an upward-facing telescope with accurate angle measurement scales. A star close to the zenith of known declination from the Pole Star was used to determine latitude, as a direct measurement of the Pole Star could be affected by refraction. The Great Trigonometrical Survey of India started on 10 April 1802 with the measurement of a baseline near Madras. The East India Company thought that this project would take about five years, but it took nearly 70 years, well past the Indian Rebellion of 1857 and the end of company rule in India. Because of the extent of the land to be surveyed, the surveyors did not triangulate the whole of India but instead created what they called a "gridiron" of triangulation chains running from north to south and east to west. The Trigonometrical Survey was conducted independently of other surveys, notably the topographical and revenue surveys. In 1875, the decision was taken that the Survey budget should be reduced from 240,000 to 200,000 pounds. This resulted in a reorganization under Surveyor-General Colonel J.T. Walker to amalgamate the Great Trigonometrical, Topographical and Revenue Surveys into the Survey of India. to elevate the instruments == Instruments and methods used ==

Triangulation surveys were based on a few carefully measured baselines and a series of angles. The initial baseline was measured with great care since the accuracy of the subsequent survey was critically dependent upon it. Various corrections were applied, principally temperature. An especially accurate folding chain was used, laid on horizontal tables, all shaded from the sun and with constant tension. The early surveys made use of large and bulky theodolites made by William Carey, a zenith sector made by Jesse Ramsden, and chains. Later surveys used more compact theodolites. in 2012 Accurate instruments could not always be purchased through the standard system of government contract, and Everest personally supervised the construction of instruments. He had a maker, Henry Barrow, set up an instrument company in Calcutta. Barrow was succeeded by Syed Mohsin from Arcot, Tamil Nadu, and after his death, the instruments were supplied by Cooke from York. Correcting deviations To achieve the highest accuracy, a number of corrections were applied to all distances calculated from simple trigonometry: • Curvature of the Earth • The non-spherical nature of the curvature of the Earth • Gravitational influence of mountains on pendulums and plumb lines • Refraction • Height above mean sea level == Superintendents ==

• 1818–1823 – William Lambton • 1823–1843 – Sir George Everest • 1843–1861 – Andrew Scott Waugh • 1861–1883 – James Thomas Walker • 1884–1888 – Charles Thomas Haig • 1888–1894 – George Strahan • 1894–1899 – St George Corbet Gore • 1899–1911 – Sidney Gerald Burrard • 1912–1921 – Sir Gerald Ponsonby Lenox-Conyngham == See also ==