Global digital elevation model (GDEM). Computer model showing the varying heights of the world's land masses. The elevations are colour coded: low=purple, medium=green and yellow, high=orange, red, and white. This GDEM was created by processing and stereo-correlating the 1.3 million archive optical images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite. ASTER is a joint program of NASA, and Japan's Ministry of Economy Trade and Industry.