Great Trigonometric Survey
The Great Trigonometric Survey was a project of the Survey of India throughout most of the 19th century. It was piloted in its initial stages by William Lambton, and later by George Everest. Among the many accomplishments of the Survey were the demarcation of the British territories in India and the measurement of the height of the Himalayan giants: Everest, K2, and Kanchenjunga. The Survey had an enormous scientific impact as well, being responsible for one of the first accurate measurements of a section of an arc of longitude, and for measurements of the geodesic anomaly.
The Great Trigonometrical Survey of India started on 10 April 1802 with the measurement of a baseline near Madras. 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. The East India Company thought that this project would take about 5 years but eventually it took more than 60 years, draining the profits of the Company, so much so it was brought under the Crown after 1857. Because of the extent of the land to be surveyed, the surveyors did not triangulate the whole of India but instead created triangulation chains running from North to South and East to West. At times the survey party numbered 700 people. 
The initial baseline was measured with great accuracy, 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 a constant tension.
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
- Height above sea level
- 1818–23 – William Lambton
- 1823–43 – Sir George Everest
- 1843–61 – Andrew Scott Waugh
- 1861–84 – James Thomas Walker
- 1885–1911 – Sidney Gerald Burrard
- 1912–21 – Sir Gerald Ponsonby Lenox-Conyngham
Land purchases by surveyors
Many surveyors became very rich. Prominent among them was Andrew Chamrette, his son Peter Chamrette, and his grandson Charles Chamrette, who worked for the GTS of India from 1802 to 1876. This family acquired in excess than 1,800 acres (7.3 km2) of land in the Kapsi, Yavatmal and Maharashtra (formerly CP Berar) districts. George Everest bought 600 acres (2.4 km2) of land near Dehra Doon.
- Matthew Edney. 1997. Mapping an Empire. Chicago: University of Chicago Press. ISBN 0-226-18488-9.
- John Keay. 2000. The Great Arc. London: Harper Collins. ISBN 0-00-257062-9.