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.[1] With the acquisition of new territory, it employed several explorers and cartographers to provide maps and other information on its territories, most notably James Rennell, from 1767 in Bengal. As Rennell proceeded to make maps, the lack of precise measurement was noticed.[1] In 1800, shortly after the Company victory over Tipu Sultan, William Lambton, an infantry soldier with experience in surveying, proposed to remedy precisely that, through a series of triangulations, initially through the newly-acquired territory of Mysore, and eventually across the entire subcontinent.[5]

Measurement of the Calcutta baseline in 1832 based on a sketch by James Prinsep. 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.[lower-alpha 1]

A Zenith sector 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.[1] Major Lambton selected the flat plains with St. Thomas Mount at the north end and Perumbauk hill at the southern end. The baseline was 7.5 miles (12.1 km) long. Lieutenant Kater was despatched to find high vantage points on the hills of the west so that the coastal points of Tellicherry and Cannanore could be connected. The high hills chosen were Mount Delly and Tadiandamol. The distance from coast to coast was 360 miles (580 km) and this survey line was completed in 1806.[7] 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.[1] At times the survey party numbered 700 people.[8]

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.[9]

Survey towers used by George Everest to elevate the instruments

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,[10] a Zenith sector made by Jesse Ramsden and 100-foot (30 m) chains. Later surveys used more compact theodolites.

One of the survey towers near Kolkata 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.[11][12]

• 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

• Principal Triangulation of Great Britain

Wikimedia Commons has media related to Great Trigonometrical Survey.

• Edney, Matthew H. (2009). Mapping an Empire: The Geographical Construction of British India, 1765-1843. University of Chicago Press. ISBN 978-0-226-18486-9.
• John Keay. 2000. The Great Arc, London: Harper Collins. ISBN 0-00-257062-9.
• Dean, Riaz (2019). Mapping The Great Game: Explorers, Spies & Maps in Nineteenth-century Asia. Oxford: Casemate (UK). pp. 67–123. ISBN 978-1-61200-814-1.
• Deb Roy, Rama (1986), "The Great Trigonometrical Survey of India in a Historical Perspective" (PDF), Indian Journal of History of Science, 21 (1): 22–32, archived from the original (PDF) on 25 January 2014
• Reginald Henry Phillimore, Historical Records of the Survey of India, 5 vols. Dehra Dun, Survey of India (1945–1968)
  • Volume 1: 18th Century; Volume 2: 1800 to 1815; Volume 3: 1815 to 1830; Volume 4: 1830 to 1843; Volume 5: 1844 to 1861