Hari Kunzru investigates the Great Arc, the 50-year project begun 200 years ago to map the Indian subcontinent to the nearest inch. Britain's lack of political control of the region added to the immensity of the task, with hundreds dying as a result of disease, adversity or local resistance. It was George Everest who saw the survey through to the end and gave his name to its greatest discovery - the highest mountain in the world.