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By Root 894 0

the area “The Great American Desert.” As a result, the plains were held to be valueless and settlers avoided them for decades. As it turned out, the explorers had visited during a dry period in the region’s drought cycle, and of course they had no idea of the vast aquifers under their feet that made the area ideal for irrigation farming. Today, the same region is called “America’s breadbasket.”

I mention these misconceptions not to discredit the early map-makers but to show what they were up against: they were writing the first records of every single thing they saw. Their horizon was only three miles away, and they had no way to transcend the limits of their own viewpoint. Consider the laborious process of making the first survey of a region using eighteenth- or nineteenth-century technology. First you need to establish a baseline—a precisely known distance between two points. Today you’d do that with a laser; measure the time it takes light to reflect off a prism, and within seconds you’d have the distance. But back then it meant inching across the countryside with a sixty-six-foot chain, moving the chain like a football referee every time it got fully extended and always taking great care to keep it straight and at a constant elevation (on wooden trestles, if necessary). Marking off a single seven-mile baseline could take weeks.

And then the fun would really start. From both ends of your baseline, you use a bulky instrument called a theodolite to measure the angle to a single landmark—a hilltop, maybe, or a distant church steeple. With a little tenth-grade trigonometry, you use the baseline length and the two angles to compute the distances from each endpoint to the third landmark. Well done! You have just surveyed a single triangle! Now take one of your endpoints and the new landmark, and make that distance the baseline of a second triangle, and one of that triangle’s sides the baseline of a third triangle, and so on. Now please try to resist blowing your brains out when I tell you that the Great Trigonometrical Survey that mapped British India two centuries ago required more than forty thousand triangles to complete and stretched from a five-year project into an eighty-year one.*

It was a Herculean task, the Indian survey. This kind of triangulation is difficult enough if you’re mapping, say, Devonshire. It’s almost inconceivable on a subcontinent of dense jungles and the world’s highest mountains,* where torrential rains might halt mapping for months at a time and you have to constantly replace the surveyors killed by malaria. Where there were no easily visible landmarks to sight to or from, rickety bamboo scaffolds would be built, and many of the flagmen stationed atop them fell to their deaths. James Rennell, the “father of Indian geography,” was almost killed on the Bhutanese border in 1776 when his small party of sepoys was attacked by hundreds of Sannyassa fakirs, who had been terrorizing local villages. Armed only with a cutlass, Rennell fought off two lines of the bandits and crawled back to the British camp, bleeding copiously from at least five sword wounds, one more than a foot long. The nearest doctor was three hundred miles away, but Rennell somehow clung to life, though he was never the same after surviving the attack. Even more remarkable is the story of Nain Singh, the Bhotian schoolteacher who spent the better part of ten years exploring the Himalayan “roof of the world” for the British. Tibet was closed to Westerners under penalty of death, but Singh was able to smuggle himself across the border and complete the five-hundred-mile trek to Lhasa, where he met the Dalai Lama himself. Singh’s Buddhist prayer wheel concealed a hidden compartment for notes and a compass; his rosary had been doctored so he could use the beads to count his paces. At every place he stopped, he would secretly use his sextant to determine latitude and boil a pot of water to measure altitude. Though he received only twenty rupees a month for his pains, his measurements formed the basis for the only maps of Tibet available for