We embark now on further exploration of what GIS actually does, how it works, what software systems are available for its use, and what typical end products and applications look like. Before moving on, you may wish to gain a thumbnail insight into GIS through two tutorials now on the Internet. (Frankly, both of these are rather generalized but you can acquire a useful overview by working through them. Overall, the writer [NMS] has found that most of the 100s of sites he visited through Internet searchs are rudimentary and highly particularized and don't offer much in the way of broad understanding of GIS .) The first site (http://info.er.usgs.gov/research/gis/title.html) has been prepared by geographers at the U.S. Geological Survey. The second (http://www.esri.com/base/users/dtgis/front.html), developed by ESRI, focuses on doing GIS analysis from desktop computers. ESRI has also produced a glossary appropriate to GIS, accessed here (http://www.esri.com/base/users/glossary/glossary.html). Two other sites worth visiting are the list of federal, state, and other organizations in the U.S. that have informative GIS sites (http://www.epa.gov/docs/oppe/spatial.html) and a good source of links to many worldwide sites prepared by ERIN, an environmental group tied to the Australian government (http://kaos.erin.gov.au/other_servers/category/Geographic_Information_Systems.html).

Perhaps the best way to appreciate the power of GIS, even before examining the design and function of the data handling system, is to introduce you to a typical case study ** and the thinking behind the steps involved in carrying out a site suitability analysis. The rationale behind such an analysis is summarized by this diagram

Suppose that three factors or variables, among the attributes that describe a geographic area under consideration, are essential in determining best sites for, say, a land development venture: Vegetation, Topography, and Soils . Each is represented by a data element - in this case a map showing the characteristics and distribution of the members or classes within the element theme, e.g., different types of soils and their properties. One, usually a cartographic map, is usually designated as the base over which the others will be overlain (each then constitutes a data layer) either manually or digitally. Now, some soils, vegetation cover types, and elevations are more favorable than others in specifying their role in the site selection process. Thus, for a certain intended use high areas are preferred over low. Relative heights can be assigned numerical ratings, say from 1 to 5. Soils with optimum drainage can be marked by higher numbers in a scale of 1 to 8. A data element map can be subdivided into cells in a grid. Each cell is assigned a value based on its thematic rating. Other kinds of data, e.g., tables representing some condition, are also amenable to incorporation in the cells, provided there is some spatial connection. When the several maps sharing the same cells, each comprising a data layer, are combined (in a modern GIS this is done digitally), the values for each cell (ranging from lowest numbers = worst suited to highest = best) are summed. The outcome is a decision whereby areas with the highest resultant scores are judged most favorably suited.

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