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Arc of triumph

How the meridian was measured 200 years ago is celebrated in a new exhibition. Douglas Blane reports

Directed by William Lambton, a mild-mannered major with a passion for science, the Great Trigonometric Survey began in Madras, East India 200 years ago. Travelling through forests and across swollen rivers, and fighting unknown fevers, the surveying teams lived through dust and heat, cyclones and monsoons, and encountered tigers, bears, snakes and scorpions in their quest to map and measure the Indian subcontinent.

The survey took almost 70 years to complete and covered 2,400km. Its bicentenary is celebrated in the Great Arc exhibition, which is touring the UK during the autumn term. The survey began on the Coromandel Coast, where surveyors spread out a 100-foot chain 400 times, to create a perfectly straight baseline approximately 12km long. From either end they took sight on a flag, using a theodolite weighing 508kg to measure the angles and create two more sides of a triangle whose calculated lengths they used as new baselines. The process was repeated thousands of times as the survey painstakingly crisscrossed the subcontinent with a great grid of topographic triangles.

Using a blend of text, music, film, computer simulation and real instruments, the Great Arc tells an enthralling tale of life at the frontier - of the known world and of science. For Lambton's ambitious aim was "to determine by actual measurement the magnitude and figure of the earth", an aim that had obsessed scientists since the Ancient Greeks.

Successful in this, the survey did far more, as the exhibition illustrates in a sequence of dramatic displays, with titles such as "Unparalleled Peaks" and "Spaceship India", which explore the human stories, scientific discoveries and geographical, historical and political significance of the survey.

Besides laying the foundations for modern India's infrastructure - roads, railways and telecommunications - the survey also discovered the source of the Ganges and the Brahmaputra, provided data for the theory of continental drift, and accurately measured the shimmering, snowcapped heights of the Himalayas.

Chief computer Radhanath Sickdhar of Calcutta performed the calculations that proved that Chomolungma, a towering but elusive presence, was the tallest mountain in the world. It was renamed Mount Everest in honour of George Everest, the second director of the survey.

The main Great Arc event will be held in London, with satellite exhibitions visiting Birmingham and Manchester. A sister exhibition will also visit towns and cities across India and a pilot project involving 500 schools will teach mapping techniques to Indian schoolchildren.

Great Arc datesSeptember 4-20: Thinktank Trust, Birmingham; October 1 - November 12: Atlantis Gallery, London; November 26 - January 15: Museum of Science and Industry, Manchester; Entrance pound;5, with concessions for school

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