Four pounds doesn't feel like a lot when it's a bag of groceries you're schlepping out to the parking lot. But if you're lugging it up to the top of Mount Everest, four extra pounds in your pack can feel like a ton. The climbers who took a four-pound GPS receiver to the summit of the world's highest mountain last year were convinced nonetheless that the extra weight would be worth the effort. That's because GPS--short for "global positioning system," a satellite-based network that uses radio signals to determine location--can gauge a mountain's height with an accuracy unequaled by standard surveying methods. Mapmakers hope the new technology will bring some finality to a field long plagued by fuzzy numbers. But like most alpine pursuits, the urge to quantify Everest is largely metaphysical: They measure it because it's there.
Mountains don't have "official" elevations, since there's no officially designated international body to sort out competing claims. And the claims do compete: Estimates of Everest's height, for example, have differed by more than 100 feet since British surveyors espied ÔPeak 15' in the mid-1800s. Those geographers instantly suspected that they were looking at the highest mountain in the world. Their calculations, made from six sites on the low and distant plains of India, put its altitude at 29,002 feet. At the turn of the century, a second survey, which added several stations in the mountains to the east, gave an elevation of 29,141 feet. When Nepal opened its borders in the 1950s, Indian surveyors were able to get closer to the mountain than ever before, and the average of their 12 readings resulted in the elevation most widely cited today: 29,028 feet.
Cartographers now have many reasons to doubt this number. Although the Indian survey was meticulous, it preceded the discovery of movements in the Earth's crust, called plate tectonics, that revolutionized studies of geological features. It turns out, for example, that the entire Himalayan range is creeping toward China at a rate of about five centimeters a year, shifting Mount Everest's latitude ever northward. And geophysicists have since devised an improved model of the shape of the Earth, called the geoid, that is crucial for determining sea level at any specified location. Given that exact measurements of both latitude and sea level are required to calculate altitude, the Indian figure has become suspect.
Meanwhile, GPS technology has turned into the darling of cartographers and geophysicists alike, because its accuracy is far less dependent on unpredictable variables such as human eyesight and hazy skies. "The precision of GPS means that, if you visit a place once, you can measure where it is for the cartographer, and if you visit it several times, you can find out how fast it's moving for the geophysicist," says Charles Corfield, science manager for the last two years of the Everest expedition. Unlike traditional surveying, however, GPS requires that you visit the place itself--no mean feat, when that place is higher than some jet flight paths.
The Everest project was born of a collaboration between one of America's preeminent cartographers, Bradford Washburn of the Boston Museum of Science, and geophysicist Roger Bilham of the University of Colorado at Boulder. Known for his definitive maps of Mount McKinley and the Grand Canyon, Washburn--who turns 90 in June--had crowned his career in the late 1980s with a detailed Everest topographical map. In the early 1990s, as GPS receivers started to shrink, Washburn's hankering to use the new technology to remeasure the grand mountain grew. He contacted Bilham, who was setting up GPS stations in India, Tibet, and Nepal to monitor earthquake hazards caused by the ongoing collision of crustal plates that had created the Himalayan range. The two secured funding, and by 1995 the first crew was crawling up the glacial sheets above base camp.
Only one GPS receiver is necessary for determining location, but pinpointing altitude is a bit trickier. To get a good, fast fix on the summit position, the Everest climbers had to coordinate their GPS readings with those of two other receivers: one running near base camp at 18,000 feet, and one perched on a 26,000-foot saddle called the South Col, a mile from the top. Determined teams of scientists, guides, and Sherpas would make five attempts in five consecutive years. But bad weather and the misfortunes of other climbers foiled their efforts until the morning of May 5, 1999, when two guides and five Sherpas reached the top after a five-day climb and ran their receiver for 56 minutes--about half an hour longer than was strictly required. "They had wonderful conditions--there was very little wind, and it was 16 degrees below zero, which is absolutely torrid for the top of Everest," says Washburn.
The reading gives the distance from the top of the mountain to the Earth's center, which GPS has helped to locate with unprecedented precision. Using the latest version of the geoid, Bilham and his colleagues then translated that number into a height above sea level. The figure they arrived at was announced in November at the annual meeting of the American Alpine Club: 29,035 feet. The margin of error is plus or minus seven feet.
Some experts have questioned the payoff of such herculean effort. "Most of us still don't really agree with this number," says Fred Blume, a colleague of Bilham's at the University of Colorado. "We're claiming that it's better, but it's only accurate to seven feet." The uncertainty is due to the geoid. Finding sea level in the Himalayan range is notoriously difficult, because it requires gravity measurements taken on location in a dense grid--an endeavor that the precipitous terrain virtually prohibits.
Then there's the snow problem. All estimates of Everest's elevation, including the latest one, have measured the height of the snowpack on the summit, not the summit itself. No one knows how deep the snowpack goes. But, says Corfield, it's thought to vary by at least three feet during the course of each year, as monsoons dump the white stuff in summer and wind scours it down all winter. To complicate matters even more, each year the Himalaya are being pushed upward about five to eight millimeters as the Indian subcontinent dives under Asia.
In the face of all this uncertainty, the GPS measurement of Everest illustrates the difficulty of deploying a precise technology in an imprecise world--a world full of seasonally variable snowpacks and sketchy gravity profiles and tectonic pirouettes. For cartographers and geophysicists alike, it represents a baseline of a sort that has never been available before, one that can serve as a point of departure for future calculations as other parameters of geophysics and cartography are refined. Scientists do know now, without a doubt, how far the snow on top of the world's highest mountain is from the center of the Earth. It's up to posterity to conquer the other obstacles of altitude: what sea level is, how deep the snow goes, where the continents are drifting, and how fast a really big mountain is reaching up to the sky.
Visit www.mountainzone.com/everest for more about the Mount Everest expedition. If you're in Boston, visit the Museum of Science to see the large-scale relief map of the mountain, created by Bradford Washburn. For information about the museum's location and hours as well as current exhibits, see www.mos.org or call 617-723-2500.