Historical satellite imagery works like a time machine that can run backwards. It freezes the surface of the Earth from decades ago in film and pixels, so today we can still make out the old river network, field ridges, wharves and patches of housing exactly as they were. For a city that changes as fast as Guangzhou, 1980 imagery is especially precious: back then the banks of the Pearl River had no forest of high rises, city blocks and farmland were knitted together, and many waterways, villages and industrial zones that are gone today were still in place. Such imagery is widely used to study urban expansion, coastline and shoreline change, land use change detection, historical map correction, place name verification, and old town preservation and planning comparison. With it we can precisely answer "what used to be here", and give today's urban renewal a more reliable basis - which is exactly the irreplaceable value of historical remote sensing.

The images below are 1m resolution satellite images taken by the Keyhole satellite in 1980. These images come from the US Keyhole reconnaissance satellite program. At the time the film was returned to Earth, and after scanning and geometric correction it became a usable historical base map. A 1m resolution is enough to see road alignments, bridge locations and building patterns, making it an excellent source for studying more than four decades of urban change in Guangzhou.