The arrival of cheap, powerful computers over the last few decades has enabled the development of innovative software applications for storing, analyzing, and displaying geographic data. Many of these applications belong to a group of software known as a Geographic Information System (GIS). Many definitions have been proposed for what constitutes a GIS. Each of these definitions corresponds to the particular task being performed. Instead of repeating each definition, we should broadly define GIS according to what it does. Thus, the activities typically carried out on a GIS include: 

• The measurement of natural and human-made phenomena and processes from a spatial perspective. These measurements emphasize three types of properties commonly associated with systems: elements, attributes, and relationships.
  
  
• The storage of measurements in digital form in a computer database. These measurements are often linked to features on a digital map. These features can be of three types: points, lines, or polygons (areas).
  
  
• The analysis of collected measurements produces additional data and reveals new relationships by numerically manipulating and modeling different data sets.
  
  
• The depiction of the measured or analyzed data in some display - maps, graphs, lists, or summary statistics.
  
  

Components of a GIS 

            A Geographic Information System combines computer cartography with a database management system. Figure 2.34 describes some of the major components of GIS. This diagram suggests that a GIS consists of three subsystems: (1) an input system that allows for the collection of data to be used and analyzed for some purpose; (2) computer hardware and software systems that store the data, allow for data management and analysis and can be used to display data manipulations on a computer monitor; (3) an output system that generates hard copy maps, visual images, and other types of output.

    

            Two basic types of data are typically entered into a GIS. The first type of data consists of natural phenomena and features with some spatial dimension. Usually, these data elements are depicted mathematically in the GIS as points, lines, or polygons that are referenced geographically (orgeocoded) to some coordinate system. Spatial data is entered into the GIS by scanners, digitizers, GPS devices, aerial photos, and satellite imagery. The other type of data is sometimes called an attribute. Attributes are pieces of data associated with points, lines, or polygons in a GIS. This attribute data can be analyzed to identify important patterns. Attribute data is entered directly into a database associated with element data.

    

            The difference between element and attribute data can be illustrated in Figures 2.35 and 2.36. Figure 2.35 shows the locations of some earthquakes that have occurred in the last century. These plotted data points can be considered elements because their primary purpose is to describe the locations of earthquakes. For each earthquake plotted on this map, the GIS also contains data on its depth. These measurements can be considered attribute data because they are associated with the plotted earthquake locations in Figure 2. Figure 2.36 shows the attribute earthquake depth organized into three categories: shallow, intermediate, and deep. This analysis indicates a possible relationship between earthquake depth and spatial location: deep earthquakes do not occur at mid-oceanic ridges.

            Within the GIS database, individuals can enter, analyze, and manipulate data associated with real-world spatial elements. The GIS cartographic software enables one to display geographic information at any scale or projection, and to view it as a variety of layers that can be turned on or off. Each layer would show a different aspect of a place on the Earth. These layers could show a road network, topography, vegetation cover, streams, water bodies, or the distribution of annual precipitation. The output illustrated in Figure 2.37 merges data layers for vegetation community type, glaciers and ice fields, and water bodies (streams, lakes, and oceans). 

FIGURE 2.35  Global distribution of earthquake events that have occurred over the last century.  Image Copyright: Michael Pidwirny.

FIGURE 2.36  Earthquake events organized according to depth (yellow = surface to 25 kilometers below the surface, red = 26 to 75 kilometers below the surface, and blue = 76 to 660 kilometers below the surface).  Image Copyright: Michael Pidwirny.

FIGURE 2.37  Graphic output from a GIS. This GIS contains information about the major plant communities, lakes and streams, and glaciers and ice fields found occupying the province of British Columbia, Canada. The output shows southern Vancouver Island and part of the mainland of British Columbia.  Image Copyright: Michael Pidwirny.