It's a truism among specialists in geographic information that the lion's share of the cost of most GIS projects is associated with the development and maintenance of a suitable database. It seems appropriate, therefore, that our first course in geographic information systems should focus upon the properties of geographic data.
I began this first chapter by defining data in a generic sense, as sets of symbols that represent measurements of phenomena. I suggested that data are the raw materials from which information is produced. Information systems, such as database management systems, are technologies that people use to transform data into the information needed to answer questions and to make decisions.
Spatial data are special data. They represent the locations, extents, and attributes of objects and phenomena that make up the Earth's surface at particular times. Geographic data differ from other kinds of data in that they are distributed along a continuous, nearly spherical globe. They also have the unique property that the closer two entities are located, the more likely they are to share similar attributes.
GIS is a special kind of information system that combines the capabilities of database management systems with those of mapping systems. GIS is one object of study of the loosely-knit, multidisciplinary field called Geographic Information Science and Technology. GIS is also a profession--one of several that make up the geospatial industry. As Yogi Berra said, "In theory, there's no difference between theory and practice. In practice there is." In the chapters and projects that follow, we'll investigate the nature of geographic information from both conceptual and practical points of view.