Geospatial Imagery

Geospatial imagery depicts the locations and characteristics of features, both natural and constructed, on Earth's surface. The general term can encompass aerial photographs, multispectral or hyperspectral images based the response of a sensor to specific portions of the electromagnetic spectrum, shaded relief images produced from digital elevation models, and maps. Geospatial imagery can be combined with other kinds of information, for example databases showing the addresses of convicted criminals or hazardous chemical storage facilities, within geographic information system (GIS) software to visualize spatial patterns that may be important in civil and criminal investigations.

The first widely used type of geospatial imagery was aerial photography, which can now be obtained using cameras mounted in aircraft, spacecraft, or satellites. If the photographs are taken from a spacecraft or satellite, however, they are likely to be described as space or satellite images in order to distinguish them from photographs taken from aircraft. Aerial photographs can be taken either vertically or obliquely, and overlapping photographs taken from slightly different positions can be viewed stereoscopically in order to emphasize topographic features. Multispectral or hyperspectral imagery is created using instruments that are sensitive to specific portions of the electromagnetic spectrum, including portions that lie beyond the range of human vision. Multi-spectral images typically consist of several bands or ranges of information (in many cases infrared, red, blue, and green bands). Hyperspectral imagery, in contrast, can consist of 200 or more bands and can be processed to emphasize the occurrence of specific minerals or plant types. Bands that fall outside the range of human vision must be assigned visible colors in order to be seen, and the resulting images are known as false-color images. Like aerial photographs, multispectral and hyperspectral images can be obtained from instruments in aircraft, spacecraft, or satellites.

Publicly available geospatial imagery obtained from space was, for many years, not detailed enough to be used in most criminal and civil investigations because its resolution was too low. Landsat satellites launched in the 1970s, for example, had a maximum resolution of 30 meters per pixel. Because it takes many pixels to create a recognizable image of an object such as a building or an automobile, the smallest features than can be clearly seen in an image are many times larger than the maximum resolution. As of 2005, some modern commercial satellites had panchromatic (black and white) image resolution of less than 1 meter per pixel, which is at the limit of utility for forensic investigations in which individual buildings or vehicles must be identified.

One well known forensic investigation in which geospatial imagery played an important role was the search for the body of Xiana Fairchild, a 7-year-old girl who disappeared from her California home in late 1999. Her skull was found and identified using DNA analysis more than a year later, and investigators requested detailed maps and aerial photographs that could be used in the search for her body. Digital orthophotoquads, which are electronic versions of aerial photographs that are corrected to remove distortion (orthorectified) and then referenced to map coordinates, were provided to searchers for use on laptop computers in the field. Although her body was never found, a suspect was arrested and charged in 2004.

After it was refused permission to revisit a Dow Chemical plant after an initial inspection during the 1970s, the U.S. Environmental Protection Agency (EPA) used aerial photographs to monitor activity at the facility. Although the company had concealed its activities from observers at ground level, many parts of the facility were visible from the air. The company argued that aerial photography constituted an illegal search that violated the Fourth Amendment of the U.S. Constitution, but the Supreme Court ultimately ruled in favor of the EPA. This case is often cited as a precedent because it gave a government agency the authority to use geospatial imagery to monitor potentially illegal activities. In a different case, related to a Superfund pollution cleanup investigation, the Nutra Sweet Company used a series of aerial photographs to show that contaminants had been dumped on nearby land owned by the X-L Engineering Company and transported beneath Nutra Sweet's property by groundwater. The photographs were one piece of information used to establish that X-L Engineering, not Nutra Sweet, was responsible for the groundwater contamination.

Geospatial imagery can also be used to resolve unsettled questions about international atrocities such as the Katyn Forest Massacre, in which 4500 Polish officers and soldiers were killed during the early days of World War II. German forces discovered mass graves near the Russian city of Smolensk in 1943, and the German government accused the Soviet government of mass murder. Soviet leader Josef Stalin refuted the charge and accused Germany of the atrocity. Despite evidence suggesting otherwise, the United States and Great Britain accepted Stalin's explanation and resisted further investigations. Aerial photographs taken by the German air force (Luftwaffe) during the war, which were captured and held as classified documents by the U.S. National Archives until 1979, provided important evidence in the form of images taken before, during, and after the area was occupied by German forces. A set of aerial photographs taken by the Germans in 1944, after the area had been recaptured by the Soviets, showed the bodies being removed and evidence of the massacre being destroyed by Soviet bulldozers. More recently, satellite images showing destroyed villages were used to assess the effects of civil unrest and document possible genocide in the Darfur region of Sudan in 2004.