Many people think of GIS as a computer tool for making maps. Actually, it is a complex technology beginning with the digital representation of landscapes captured by cameras, digitizers, or scanners, in some cases transmitted by satellite, and, with the help of computer systems, stored, checked, manipulated, enhanced, analyzed, and displayed as data referenced to the earth. This spatial information includes earth coordinates and geometric and topological configurations to portray spatial relationships between features such as streams, roads, cities, and mountains. GIS is "a digital representation of the landscape of a place (site, region, planet), structured to support analysis." Under this broad definition, GIS conceivably may include process models and transport models as well as mapping and other spatial functions. The ability to integrate and analyze spatial data is what sets GIS apart from the multitude of graphics, computer-aided design and drafting, and mapping software systems.
Typical sources of geographic data for computer manipulation include digitized maps, field survey data, aerial photographs (including infrared photographs), and satellite imagery. Most image data are collected using remote sensing techniques. Aerial photographs are normally taken with special mapping cameras using photographic film. Most commercially available satellite imagery is collected using multispectral scanners, which record light intensities in different wavelengths in the spectrum--from infrared through visible light through ultraviolet light.
Spatial information can be represented in two distinctly different forms. Satellite images, for example, usually appear as raster data, a gridded matrix in which the position of each data point is indicated by its row and column numbers. Each position on a computer screen or map thus corresponds to the position on the ground measured by the satellite as it passes overhead. In contrast, cartographic features such as roads, boundaries, buildings, and contour lines usually are represented in vector form. In digitizing a lake, for example, the shoreline can be indicated as a series of points and line segments. In this case, each point is measured in Cartesian (X, Y) coordinates and each line segment is measured as a vector leading from one point to the next. The more points recorded, the more detailed the shoreline will be. Both forms, raster and vector, are essential to support environmental restoration projects on the ORNL reservation, for instance, and the software must be capable of rapid conversion from one form to the other.
For such geographic information to be meaningful, it must be accompanied by "metadata" documenting the source, description, specifications, accuracy, time of acquisition, and quality of each data element. As GIS technologies and multitudes of geographic data bases have spread to the desktop in the past decade, metadata have become very important. Good metadata are essential in determining fitness of the geospatial data for each intended use--that is, determining which applications can be accomplished while ensuring the desired quality of results and decisions made from those data.
This definition provided by The Oak Ridge National Laboratory
This definition provided by INTRODUCING
GIS OUTLINE at USC
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