A geographic information system (GIS) is basically a computer-based system capable of storing virtually any information that could be found on a paper map. It can, however, be much more useful than a paper map. A GIS has several advantages including the ability to display maps on a computer screen as well as detailed information about map features such as highways, cities, parcels, and buildings. Map features are usually stored in a data structure that consists of layers, often called coverages. As shown in the figure below, the map of Illinois was constructed by combining three separate coverages: Coverage containing Congressional Districts Coverage with Amtrak stations Coverage with major highways The best advantage of a GIS is its ability to quickly search and analyze map features and attributes in a way that would be virtually impossible with traditional paper maps. For example, you could point to a particular highway with your mouse and click to find out the name of the highway, how many lanes wide it is, and the type of pavement used. If you were looking at a map highlighting state parks, clicking the mouse button on a park could let you view a picture of that park's entrance. As another practical example of the power of a GIS, imagine that you are in the real estate business. A newly developed subdivision has a variety of lots available for purchase. An interested client has asked you to help locate all lots with area greater than 15,000 square feet having an assessed valuation less than $40,000. With an appropriate GIS you could quickly obtain a map like the one below. In this map, all parcels that are colored pink and cross-hatched in blue meet your client's criteria. Municipalities, counties, and cities throughout the United States have become ardent users of GIS technology. The tremendous cost of converting stacks of paper maps into a computer-based GIS (i.e., digitizing) has been the only major disadvantage noted by GIS developers. Although great improvements have been made recently to speed up the process of digitizing, it is still a very tedious process that requires much human-machine interaction.