The GAP program uses thematic mapper images from NASA's Landsat Program as the starting point for the vegetation and landuse maps. These images are transmitted from satellites circling the planet in overlapping orbits between the North and South Poles. The images are transmitted to 16 stations worldwide and then sent to Maryland for preprocessing. For example, they are screened for cloud cover. The complete set of images, archived in the US and at other sites around the world, represents a unique resource for global change researchers.