aerial and satellite photography

aerial and satellite photography, technology and science of taking still or moving-picture photographs from a camera mounted on a balloon, airplane, satellite, rocket, or spacecraft. In the 19th cent., photographers such as Thaddeus Lowe and George R. Lawrence took impressive pictures with cameras suspended in hot-air balloons or hung from kites, demonstrating both the scenic and military value of aerial photography. With the development of aviation, photogrammetry (the science of making measurements and maps from photographs) became an important tool. During World War I and subsequent conflicts, aerial photographs provided vital intelligence. Military aerial photography has now advanced to the point that the rank of a foot soldier can be determined from photographs taken from high-flying planes and satellites. Because of its military importance, much of the most sophisticated surveillance technology remains classified.

Aerial photography and satellite photography work in similar fashion. Course and speed are set before entering the area to be photographed, to ensure uniformity of speed and altitude. The result is an image of a narrow strip, which can be combined with overlapping images of neighboring strips to produce a panoramic view, commonly called a mosaic. Commercially available aerial and satellite photographs are capable of resolving objects of about 10 sq ft (1 sq m), which means that a satellite would be able to distinguish between a car and truck. Aerial photographs may be high oblique (including the horizon), low oblique (below the horizon), or vertical (perpendicular to the earth). Only the vertical may be accurately scaled for mapmaking purposes. Often a multilens camera is used to photograph one section vertically and the adjacent areas obliquely. The individual oblique exposures are then corrected, scaled, and joined to the vertical section to form one continuous photograph. By viewing two overlapping photographs through a stereoscope, a three-dimensional image of a region, or topographic map, can be obtained.

Images can also be produced at other wavelengths, such as microwave or infrared, by using a technique known as remote scanning, which measures variations in spectral reflectance rather than patterns of light and shadow. Remote scanning aids such disparate fields as archaeology, geology, forestry, highway construction, and land conservation. The best-known remote scanners are the Landsat series of satellites, which have mapped vegetation and geological formations on the earth's surface since 1972; the French SPOT series, first launched in 1986; Magellan, which used radar to map the planet Venus (1990); Lunar Prospector, which mapped the moon's surface composition and its magnetic and gravity fields (1998); Mars Global Surveyor, which engaged in a systematic mapping of Mars (1999); and Galileo, which returned pictures of Jupiter and its major moons (1995–2003).

See P. R. Wolf, Elements of Photogrammetry (1983); H. Lloyd, Aerial Photography (1990); R. H. Arnold, Interpretation of Airphotos and Remotely Sensed Imagery (1995); N. Henbest, The Planets: Portraits of New Worlds (1995); E. D. Conway, An Introduction to Satellite Image Interpretation (1997); P. Taubman, Secret Empire: Eisenhower, the CIA and the Hidden Story of America's Space Espionage (2003).

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