Trigonometrical Survey
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     The Great Trigonometrical Survey in India began in 1802 when William Lambton laid the baseline from St. Thomas Mount 12 km to a hillock near Pallavaram to determine the length of a diegree of latitude along a longitude in the middle of the sub-continent. Lambton and George Everest spent the next 50 years creating a geometric web of trianulations, 2400 km long, across the length of the sub-continent, the "Great Indian Arc of the Meridian." The purpose was to form the basis for an accurate survey of India, and in addition to determine by actual measurement the magnitude and figure of the earth. The Survey is commemorated by the stylized map of India on the middle stamp.

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     Nain Singh (1826-82) and Radhanath Sikdar (1813-1870) are also pictured on the souvenir sheet. Singh explored the Himalayas, mapped the trade route to Tibet, mapped a large part of the Tsangpo River in Tibet, and determined the altitude and coordinates of Lhasa.
     Sikdar (1813-1870) joined the Survey in 1840. He calculated the height of Peak XV (later renamed Mt. Everest), and determined that it was the highest mountain in the world.