Dec. 8, 2004 — Scientists have surveyed the flattest place on Earth with some unusual technology, and they've found it pretty darn flat.

The Salar de Uyuni on the Altiplano of southwestern Bolivia is the world's largest salt flat, measuring 3,800 square miles, about half the size of the Hawaii or New Jersey.

Using specialized satellite imagery, researchers from NASA and the Scripps Institution of Oceanography were able to find that elevation varied by less than two feet.

"This is the flattest piece of real estate I've ever seen," said geophysicist Bruce Bills of NASA's Goddard Flight Center. "It's a perfectly flat, white parking lot."

Bills and his colleagues used satellite images from NASA's Terra satellite's Multi-angle Imaging SpectroRadiometer (MISR) to look at the Salar de Uyuni when it was dry, and also when it was flooded with several inches of rainwater, particularly in El Niño years.

Since the water surface on the Salar de Uyuni on a windless day is the epitome of flat — except for the curve of the Earth — the researchers could use the way that sunlight reflecting off the bright salt was dimmed by small differences in water depth to reveal small variations in depth.

What they found is that over the vast salt surface, elevation varied by only about 16 inches.

Bills is scheduled to present the survey results at the meeting of the American Geophysical Union in San Francisco next week.

"It's an unusual, unique one," said MISR principal investigator David Diner of NASA's Jet Propulsion Lab, of the use of MISR to survey the Salar de Uyuni. "But it's great to see."

Diner hopes Bills and his colleagues follow up with the use of MISR images of the Salar de Uyuni taken from angles — which catches reflected sunlight coming through the water at an angle. That might provide a way to confirm the initial flatness measurements, he said.

As for why the Salar de Uyuni is so flat, Bills said it was on part because of the occasional flooding. The floods act like a Zamboni machine does on ice: wearing down the high spots and filling in the low spots.

A case in point was a pit left by a drilling project in 2000. “A year later it was almost healed. It's a self-leveling, self-healing surface,” Bills said.

Still unexplained, however, are some broad, subtle, bumpy wave-like features only visible from space.

"If you are standing there you can't see them," Bills said. "How they form is to me still a bit of a mystery."