THE GREAT ARC MEASUREMENT

The Great Arc: The Dramatic Tale of How India Was Mapped and Everest Was Named-John Keay

In the first half of the nineteenth century a mammoth survey of India mapped, through progressive triangulation, the Great Indian Arc of the Meridian, from Cape Comorin in the south to the foothills of the Himalayas, along with several side “longitudinals”. Taking thousands of lives and costing millions of pounds over nearly fifty years, this was at the time perhaps the biggest scientific project ever undertaken. The surveyors used hills or temples or specially built towers as observation points; they cleared jungle and levelled hilltops and villages to open lines of sight; they chose the season and time of day to optimise visibility; and they were attacked by malaria, tigers, and human enemies. Trying to keep the errors over hundreds of miles down to a few inches required extraordinary care in the development, testing, and use of instruments, coupled with complex and laborious mathematics. continue

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