NOT FLAT AFTER ALL. A field survey of Bolivia's Salar de Uyuni, the world's largest salt flat, found a broad ridge (green area running southwest to northeast) flanked by shallow depressions (dark-blue areas). Red denotes higher terrain in the northeastern part of the playa.Borsa, et al.

People have long known that the world isn't flat. Now, an innovative field survey of the world's largest salt flat—a New Jersey–size playa high in the Andes—reveals that the barren expanse unexpectedly has minuscule variations in topography.

Previous field surveys of southwestern Bolivia's Salar de Uyuni hadn't detected any variations in elevation, says Adrian A. Borsa, a geophysicist at the Scripps Institution of Oceanography in La Jolla, Calif. "On topographical maps, the entire region lies in between contour lines," he notes. Even data gathered during the Shuttle Radar Topography Mission in 2002 (SN: 2/23/02, p. 126) weren't accurate enough to discern the subtle ridges and valleys of the 9,000-square-kilometer region.

Scientists have used the large, ostensibly flat area as a target when calibrating altitude-measuring instruments on Earth-orbiting satellites. In fact, Borsa and his colleagues conducted their survey to obtain accurate altitude measurements that could be used to calibrate the laser altimeter on board ICESat, an Earth-orbiting craft designed to monitor ice sheets (SN: 12/24&31/05, p. 418).

The researchers surveyed a 45-by-54-km portion of the playa in September 2002 by crisscrossing its surface in SUVs with Global Positioning System receivers and antennas strapped to their roof racks. Driving back and forth along roughly east-west and north-south paths spaced about 2.25 km apart, the scientists collected GPS data every 100 meters or so, says team member Helen A. Fricker, a geophysicist at Scripps.

The team hadn't originally expected to detect any variations in the topography of the Salar de Uyuni, says Borsa. However, their equipment proved so sensitive that the researchers could even tell that their SUVs rode about 5 millimeters higher off the ground at the end of the day, with empty gas tanks, than they did in the morning.

Analyzing the survey results, the team detected a broad ridge, highest at its ends, running from southwest to northeast. The playa's highest and lowest points lay more than 50 km apart, and their difference in elevation measured just 77 centimeters, Fricker says. The team reports its findings online and in an upcoming Geophysical Journal International.

"These findings are pushing the GPS technology to its limits," says Mauri McSaveney, a geomorphologist with GNS Science in Lower Hutt, New Zealand.

The salt flat's highs and lows roughly correspond to peaks and troughs in the local gravity field, says Borsa. That agreement suggests how the Salar de Uyuni maintains its topography, he notes. The rains that briefly flood the playa each year dissolve its surface layer of salt, flow across the ground in response to the local gravity field, and then leave the salt in place when they evaporate. If it weren't for variations in the strength of the local gravity field—and the curvature of Earth's surface—the salt flat would be truly flat.

Results of the newly reported fieldwork cut in half the estimated uncertainty in ICESat's altitude data, says James Abshire, a remote-sensing scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. As a result, researchers are now better able to detect thinning of ice sheets and glaciers. Scientists can also more easily discern the rise and fall of the surfaces of ice sheets due to the flow of water into and out of subglacial lakes (SN: 3/3/07, p. 142), he notes.

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