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Oh! Maps

I love maps. A great book on maps and mapping is Simon Berthon and Andrew Robinson’s The Shape of the World (ISBN: 0788164635). Especially fascinating is the chapter “Measuring India.”

It took about 50 years for the British to complete their survey of India. (For much of the time, incidentally, the chief surveyor was Sir George Everest.) Back then mapping was done by triangulation—surveyors hauled a half-ton theodolite through forest and across desert and up mountains and to the top of temple gateways, and measured the angle between where they’d come from and where they were going to. Doing this many times got them lots and lots of triangles; from the triangles they could figure out where things were with trigonometry. Every so often they carefully measured the distance between two points on the ground, and compared the actual distance with the calculated distance:

When Everest reached Calcutta in late 1830 the end of this 600-mile (970-km) longitudinal series of triangles lay not far short of the city. This called for the laying of a new baseline as a check against the length of the final triangular side computed from Sironj. Here was Everest’s first real chance to show off his new compensation bars. He selected a site on a road north of Calcutta and ordered two observation towers built that would be visible from the last triangle in the series. Then he called the interested citizens of Calcutta to watch the Survey in action. An elegant breakfast was spread out under tents, for consumption after the ceremonies. …

The new bars performed excellently, in fact almost too well. To Everest’s dissatisfaction they showed a difference of 7ft 11in (2.4m) between computed and measured lengths, and a discrepancy of 200ft (61m) in height. For a professional craving precision this was serious, but Everest was unable to persuade the Company, either in London or Calcutta, that it mattered. Frustrated, he vowed to make the next stage of the Survey the acme of accuracy. (p. 142)

In other words, Everest was distraught because, after triangulating his way across half of India, he was out by 2.4m horizontally, and 61m vertically.