A geographic information system (GIS) is a specialized form of an information system. In the strictest sense, it is a computer system capable of assembling, storing, manipulating, and displaying geographically-referenced information in a relational database, i.e. data identified according to their locations. Practitioners also regard the total GIS as including operating personnel and the data that go into the system.
Geographic information systems technology can be used for scientific investigations, resource management and development planning. For example, a GIS might allow emergency planners to easily calculate emergency response times in the event of a natural disaster, or a GIS might be used to find wetlands that need protection from pollution.
If you could relate information about the rainfall of your state to aerial photographs of your county, you might be able to tell which wetlands dry up at certain times of the year. A GIS, which can use information from many different sources in many different forms, can help with such analyses. The primary requirement for the source data consists of knowing the locations for the variables. Location may be annotated by x,y, and z coordinates of longitude, latitude, and elevation, or by such systems as ZIP codes or highway mile markers. Any variable that can be located spatially can be fed into a GIS. Several computer databases that can be directly entered into a GIS are being produced by government agencies and non-government organizations. Different kinds of data in map form can be entered into a GIS.
A GIS can also convert existing digital information, which may not yet be in map form, into forms it can recognize and use. For example, digital satellite images generated through remote sensing can be analyzed to produce a map-like layer of digital information about vegetative covers.
Likewise, census or hydrologic tabular data can be converted to map-like form, serving as layers of thematic information in a GIS.
Electronic scanning devices will also convert map lines and points to digits.
A GIS can be used to emphasize the spatial relationships among the objects being mapped. While a computer-aided mapping system may represent a road simply as a line, a GIS may also recognize such a road as the border between wetland and urban development, or as the link between Main Street and Blueberry Lane.
Data capture - putting the information into the system - consumes much of the time of GIS practitioners. Identities of the objects on the map must be specified, as well as their spatial relationships. Editing of automatically captured information can also prove difficult. Electronic scanners record blemishes on a map just as faithfully as they record the map features. For example, a fleck of dirt might connect two lines that should not be connected. Extraneous data must be edited, or removed from the digital data file.
Projection is a fundamental component of map making. A projection is a mathematical means of transferring information from the Earth's three-dimensional curved surface to a two-dimensional medium - paper or a computer screen. Different projections are used for different types of maps because each projection particularly suits certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.
Since much of the information in a GIS comes from existing maps, a GIS uses the processing power of the computer to transform digital information, gathered from sources with different projections, to a common projection.
Raster data files can be manipulated quickly by the computer, but they are often less detailed and may appear less visually appealing than vector data files, which can approximate the appearance of more traditional hand-drafted maps. Vector digital data have been captured as points, lines (a series of point coordinates), or areas (shapes bounded by lines). In addition, raster data files require some means of tying them to the Earth's surface; these vary from vendor to vendor, but a widely-used way of doing this is through a World file which represents the location, scale and rotation of the map image.
An example of data typically held in a vector file would be the property boundaries for a housing subdivision.
Data restructuring can be performed by a GIS to convert data into different formats. For example, a GIS may be used to convert a satellite image map to a vector structure by generating lines around all cells with the same classification, while determining the cell spatial relationships, such as adjacency or inclusion.
Thus a GIS can be used to analyze land use information in conjunction with property ownership information.
What do you know about the swampy area at the end of your street? With a GIS you can "point" at a location, object, or area on the screen and retrieve recorded information about it from off-screen files.
Using scanned aerial photographs as a visual guide, you can ask a GIS about the geology or hydrology of the area, or even about how close a swamp is to the end of a street. This kind of analytic function allows you to draw conclusions about the swamp's environmental sensitivity.
35,000 years ago, on the walls of caves near Lascaux, France, Cro-Magnon hunters drew pictures of the animals they hunted. Associated with the animal drawings are track lines and tallies thought to depict migration routes. These early records followed the two-element structure of modern geographic information systems: a graphic file linked to an attribute database.
Today, biologists use collar transmitters and satellite receivers to track the migration routes of caribou and polar bears to help design programs to protect the animals. One GIS displayed the migration routes by different colors for each month for 21 months. Researchers then used the GIS to superimpose the migration routes on maps of oil-development plans to determine the potential for interference with the animals.
Maps have traditionally been used to explore the Earth and to exploit its resources. GIS technology, as an expansion of cartographic science, has enhanced the efficiency and analytic power of traditional mapping. Now, as the scientific community recognizes the environmental consequences of human activity, GIS technology is becoming an essential tool in the effort to understand the process of global change. Various map and satellite information sources can combine in modes that simulate the interactions of complex natural systems.
Through a function known as visualization, a GIS can be used to produce images - not just maps, but drawings, animations, and other cartographic products. These images allow researchers to view their subjects in ways that literally never have been seen before. The images often are equally helpful in conveying the technical concepts of GIS study-subjects to non-scientists.
