Geographic Information System

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Computer-based technologies for the storage, manipulation, and analysis of geographically referenced information. Attribute and spatial information is integrated in geographic information systems (GIS) through the notion of a data layer, which is realized in two basic data models: raster and vector. The major categories of applications comprise urban and environmentalinventory and management, policy decision support, and planning; engineering and defense applications; and scientific analysis and modeling.

A geographic information system differs from other computerized information systems in two major respects. First, the information in this type of system is geographically referenced (geocoded). Second, a geographic information system has considerable capabilities for data analysis and scientific modeling, in addition to the usual data input, storage, retrieval, and output functions.

A geographic information system is composed of software, hardware, and data. The notion of data layer (or coverage) and overlay operation lies at the heart of most software designed for geographic information systems.

Two fundamental data models, the vector and raster models, embody the overlay idea in geographic information systems. In a vector geographic information system, the geometrical configuration of a coverage is stored in the form of points, arcs (line segments), and polygons, which constitute identifiable objects in the database. In a raster geographic information system, a layer is composed of an array of elementary cells of pixels, each holding an attribute value without explicit reference to the geographic feature of which the pixel is a part. See also Computer graphics; Electronic display; Image processing.

A data layer or coverage integrates two kinds of information: attribute and spatial (geographic). The functionality of a geographic information system consists of the ways in which that information may be captured, stored, manipulated, analyzed, and presented to the user. Spatial data capture (input) may be from primary sources such as remote sensing scanners, radars, or global positioning systems, or from scanning or digitizing images and maps derived from remote sensing. Output (whether as a display on a cathode-ray tube or as hard copy) is usually in map or graph form, accompanied by tables and reports linking spatial and attribute data. The critical data management and analysis functions fall into four categories: retrieval, classification, and measurement; overlay functions; neighborhood operations; and connectivity functions. See also Digital computer; Remote sensing.

Business applications of geographic information systems are increasingly widespread and include market analysis, store location, and agribusiness (for example, determining the correct amount of fertilizers or pesticides needed at each point of a cultivated field). Engineers use geographic information systems when modeling terrain, building roads and bridges, maintaining cadastral maps, routing vehicles, drilling for water, determining what is visible from any point onthe terrain, integrating intelligence information on enemy targets, and so forth. Such applications have been facilitated through the integration of geographic information systems with global positioning systems. See also Coordinate systems; Satellite navigation systems.

Among the earliest and still most widespread applications of the technology are land information and resource management systems (for example, forest and utility management). Other commonuses of geographic information systems in an urban policy context include emergency planning, determination of optimal locations for fire stations and other public services, assistance in crime control and documentation, and electoral and school redistricting. Uses of geographic information systems have spread well beyond geography, the source discipline, and now involve most applied sciences, both social and physical, that deal with spatial data. The nature of the applications of geographic information systems in these areas ranges from simple thematic mapping for illustration purposes to complex statistical and mathematical modeling for the exploration ofhypotheses or the representation of dynamic processes.

Are integrated, spatial, data-handling programmes which will collect, store, and retrieve spatial data from the real world. They are powerful tools in decision-making, as they can incorporate co-ordinated data. It should be noted, however, that GIS only contain selected data; solely the properties which investigators have considered relevant, so that many variables will not be fed into the systems. see Geographical Analysis Machine, geography Area Planning System.