█ WILLIAM C. HANEBERG
Geospatial imagery encompasses a wide range of graphical products that convey information about natural phenomena and human activities occurring on Earth's surface. The term can include color and panchromatic (black and white) aerial photographs, multispectral or hyperspectral digital imagery (including portions of the electromagnetic spectrum that lie beyond the range of human vision), and products such as shaded relief maps or three-dimensional images produced from digital elevation models. A related term, geospatial intelligence, describes the use of geospatial imagery for intelligence, security, or defense purposes.
The earliest form of geospatial imagery was aerial photography, which consists of photographs taken from an airborne or spaceborne camera. Aerial photographs can be taken either vertically, which is preferred if the photographs are to be used to prepare maps of an area, or obliquely. Overlapping vertical aerial photographs can be viewed stereoscopically to obtain a three-dimensional effect that can be useful for topographic or geologic analysis, and also used to create topographic maps. Another common form of geospatial imagery is the multispectral or hyperspectral image, which can resemble a color photograph. Instead of being created by the interaction of visible spectrum light with chemicals, however, modern multispectral and hyperspectral images are created by measuring the response of an electronic sensor to a particular portion, or band, or the electromagnetic spectrum. The bands sampled by a sensor can extend far beyond the portion of the spectrum visible to the human eye; hence multispectral and hyperspectral imagery has the potential to convey much more information than a traditional photograph. Whereas multispectral images may consist of several bands, (perhaps representing infrared, red, green, and blue light), hyperspectral images can include information from more than 200 bands. Multispectral and hyperspectral bands that fall outside the range of human vision must be assigned colors if they are to be seen by humans. The resulting images are known as false color images because their chosen colors represent the intensity of the sensor response to invisible wavelengths, not wavelengths corresponding to the colors on the printed image. Synthetic aperture radar (SAR) images consist of information obtained by instrument that actively emits a radio signal rather than passively sensing naturally reflected radiation. SAR technology can be used to generate detailed topographic maps of Earth's surface from space, even in areas covered by clouds.
The resolution of geospatial imagery has increased over time. Keyhole intelligence satellites, which have been launched by the United States since the early 1960s, currently have a resolution on the order of 2 cm (although no images of this resolution have been released to the public). The resolution of geospatial imagery currently available to the public is far less than that of classified intelligence imagery. The Landsat 1 satellite, launched in 1972, had a resolution of 80 m. Landsat 7, launched in 1999, has resolutions of 15 m for panchromatic images, 30 m for six multispectral bands, and 60 m for its thermal band. The French SPOT 5 satellite obtains images ranging in resolution from 5 m for panchromatic to 20 m for infrared. The commercial Quickbird satellite, which was launched in 2001, provides commercially available imagery with 61 cm panchromatic and 2.44 m multispectral resolution. The commercial IKONOS satellite, launched in 1999, can produce 1 m resolution color images.
Within the United States, the National Imagery and Mapping Agency (NIMA) is the single agency that the federal government relies upon to manage the acquisition, interpretation, and dissemination of geospatial information and imagery. Although it is primarily a combat support agency within the Department of Defense, NIMA also provides support to federal policy makers and government agencies. NIMA was formed in 1996 by consolidating the Defense Mapping Agency, the Central Imagery Office, the Defense Dissemination Program Office, the National Photographic Interpretation Center along with some parts of the Defense Intelligence Agency, the National Reconnaissance Office, the Defense Airborne Reconnaissance Office, and the Central Intelligence Agency.
The collection and application of geospatial imagery in support of defense and intelligence operations is heavily dependent upon computer technology. Image processing software can be used to identify features on multispectral images according their spectral signatures. The response of a multispectral sensor to grass or trees, for example, will be different than its response to a concrete road or steel building. Other applications include the use of sharpening filters to enhance images. Geographic information system (GIS) software can be used to combine different types of imagery, for example by superimposing a multispectral image and road network map on a shaded topographic relief map.
█ FURTHER READING:
Bossler, John D., John R. Jensen, Chris McMaster, and Chris Rizos (editors). Manual of Geospatial Science and Technology. Mount Laurel, NJ: Taylor & Francis, 2001.
Campbell, James B. Introduction to Remote Sensing, 3rd edition. New York: Guilford Press, 2002.
U.S. Department of Defense. 21st Century Complete Guide to the National Imagery and Mapping Agency (NIMA): Geospatial Intelligence for National Security, Geodesy for the Layman, Combat Support, Terrain Visualization. Mount Laurel, NJ: Progressive Management, 2003.
International Society for Photogrammetry and Remote Sensing, c/o Ian Dowman, Department of Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom. < http://www.isprs.org/ .>.
National Imagery and Mapping Agency. "NIMA HOME." < http://www.nima.mil > (7 March 2003).
Short, Nicholas M., Sr. "The Remote Sensing Tutorial." NASA. October 22, 2002. < http://rst.gsfc.nasa.gov/ > (7 March 2003).
Skorve, Johnny E. "Using Satellite Imagery to Map Military Bases of the Former Soviet Union." Earth Observation Magazine. April 2002. < http://www.eomonline.com/Common/currentissues/Apr02/skorve.htm > (7 March 2003).
U.S. Geological Survey "Ask USGS: Satellite Imagery." August 19, 2002. < http://ask.usgs.gov/satimage.html > (7 March 2003).
Bomb Damage, Forensic Assessment
Cuban Missile Crisis
LIDAR (Light Detection and Ranging)
RADAR, Synthetic Aperture
U-2 Spy Plane
Unmanned Aerial Vehicles (UAVs)