geographic information system
GEOGRAPHIC INFORMATION systems are widely known by their abbreviation, GIS. The most popular definition of GIS is: a computer-assisted system for the acquisition, storage, analysis, and display of geographic data. Conceptually, GIS has been defined as the digital representation of the landscape of a place (a site, a region, a planet), structured to support analysis.
A functional GIS necessarily incorporates software capable of handling geometry, attributes, and topology. Geometry consists of points, lines, and areas that represent places and geographic features located within those places.
Two distinctly different data structures are employed, and each system must be capable of converting or integrating the other. Raster data structures consist of regular grids of intersecting lines that form rectangular cells. In raster GIS, points are represented as individual cells, lines as chains of cells, and areas as patches of cells. Raster GIS is well suited, for instance, to represent satellite imagery collected by electronic scanners or survey samples collected in regular arrays by field investigators.
Vector data structures consist of individual points plus complex geometric structures comprised of points (typically called nodes) and line segments connected by vectors. Vectors are not restricted to right angles. Point features are expressed in latitude, longitude, and elevation or some other coordinate system convertible to such Earth coordinates. They may include, for instance, the corners of land ownership parcels and the intersections of transportation lines. Line features are expressed as points (usually called nodes) connected by line segments. They may include, for instance, road center lines, flight lines, and trajectories of all sorts. Area features are expressed as points and chains of line segments that close to produce polygons. They may include, for instance, administrative areas, ownership parcels, and bounded areas such as fields or forests.
In raster and vector GIS, attributes describe the characteristics of geographic features. Physical attributes may include, for instance, material (e.g., rock, soil, water, ice), temperature, and color. Cultural attributes may include, for instance, religion, language, and ownership.
Topology (not to be confused with topography) represents angular relationships (order, adjacency, etc.) that remain constant regardless of map distortion. Topology is essential if geographic data from different sources, scales, and projections are to be combined in a single geographic analysis.
A commercial GIS primarily consists of software for handling these functions. While some GIS vendors provide sample data or even real geographic data for broad regional coverage, it is generally expected that purchasers will use the GIS software to develop databases tailored to their own locale and analytical needs.
In Western society, people are constantly confronted with practical examples of GIS, but many do not know the term or recognize its many forms. Examples of GIS and its products include automobile navigation systems, geographic profiling in criminal investigations, tax assessment systems, digital weather maps, and even the U.S. military's precision weapons employed in the war in Iraq.
GIS products are overwhelmingly beneficial to society, but certain applications raise serious concerns about privacy and control. Some products currently on the market are advertised for “human tracking” and undoubtedly will be used to monitor and control the locations of individuals. Some applications will be ethical, others will not.
The science behind GIS is geography. On most university campuses, the principal GIS courses are offered in geography departments. The development of GIS, however, has involved a much larger community of spatially oriented disciplines (e.g., cartographers, topologists, and landscape architects), domain scientists (e.g., geologists, foresters, and ecologists), and computer scientists. Broadly, these developers and practitioners refer to their field as geographic information science, often distinguished as GISci.
Geography has been practiced for at least 2,500 years, starting with the ancient Greeks, and it has always been inhibited by the enormous volumes of data required to represent landscapes and regions. GIS enables a new form of automated geography that is revolutionizing society's capacity for understanding and managing just about everything for which location matters, including everything, human or otherwise, that moves or flows.