Via my brother is this article about a vast salt flat in Bolivia being used to calibrate satellites.
A precise topographical map has been made of one of the flattest places on Earth: the salar de Uyuni, a vast plain of white cemented salt in the mountains of Bolivia. The ground survey, aided by global positioning systems (GPS), shows variations in elevation of less than a metre across an area almost half the size of Wales.
Intriguingly, the work reveals bumps in the salt that lie above lumps of dense rock buried several kilometres below, just as water will bulge over a bump on the ocean floor. Knowing exactly where these bumps lie will help researchers to use the flat as a giant calibration device for satellite-based radar and laser altimeters.
The place is huge. If the comparison to Wales doesn't mean anything to you, consider that the salar de Uyuni is 4,085 square miles:
- Washington, DC: ~70 square miles
- New York City: ~470 square miles
- Delaware: ~2,490 square miles
- Island of Hawai'i: ~4,028 square miles