Remotely-sensed imagery from aircraft and satellites represent one of the fastest-growing sources for raster GIS data. While remote-sensing technology has been around for decades, recent technological advances and legislative changes have led to an dramatic increase in the types of imagery available. In the U.S., due to recent legislative changes and the repeal of the Landsat Commercialization Act, these data can now be obtained at costs well below previous levels. Also, satellite imaging has now been around long enough to allow study of temporal changes on the land surface. For example, early Landsat images beginning in 1972 can now be compared with recent observations, providing a 25+ year record of land-use, vegetation, and urban change.
