GIS - Geographic Information System
How GIS works
| A GIS stores information about the world as a collection of thematic layers that can be linked together by geography. This simple but extremely powerful and versatile concept has proven invaluable for solving many real-world problems from modeling global atmospheric circulation to recording details of planning applications. |  |
Geographic References
Geographic information contains either an explicit geographic reference, such as a latitude and longitude or national grid coordinate, or an implicit reference such as an address, a vegetation code, a forest stand identifier, or a road name. An automated process called geocoding is used to create explicit geographic references (multiple locations) from implicit references (descriptions such as just mentioned). These geographic references allow you to locate special features, such as the vegetation type or a precipitation zone on the earth’s surface for analysis.
| Vector and Raster Models Geographic information systems work with two fundamentally different types of geographic models: the ‚vector‘ model and the ‚raster‘ model. In the vector model, information about points, lines, and polygons is encoded and stored as a collection of x,y-coordinates. The location of a point feature, such as a bore hole, can be described by a single x,y coordinate. Linear features, such as rivers and roads, can be stored as a collection of point coordinates. Polygonal features, such as river catchments or a land use classification, can be stored as a closed loop of coordinates. |
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The vector model is extremely useful for describing discrete features, but less useful for describing continuously varying features. The raster model has evolved to model such continuous features. A raster image comprises a collection of grid cells rather like a scanned map or picture. Both the vector and raster models for storing geographic data have unique advantages and disadvantages. Modern GISs are able to handle both models.
| an example for a raster image: Landsat satellite image acquired on
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© 1999 TRIANET, Program of the European
Union Socrates-Comenius
Last update on 05.05.1999 by Markus
Zapke-Gründemann