When location or position is used as a primary referencing basis for data, the data involved are known as spatial data. For example, the elevations, in feet, of the landmarks Clingman’s Dome and Newfound Gap in the Great Smoky Mountains National Park are data. If the primary referencing basis for these data is the “Great Smoky National Park,” or x miles south of Gatlinburg, Tennessee, on U.S. Highway 441, or p degrees latitude and q degrees longitude, then the elevations could be referred to as spatial data.
Spatial data, then, are discrete symbols (numbers, letters, or special characters) used to describe some entity; these data are organized according to the location of that entity in the three-dimensional world. They are data that pertain to the space occupied by objects. It includes cities, rivers, roads, states, crop coverages, mountain ranges, and so on.
Normally, when it is desirable to describe things in the real world by spatial data, the objects are abstracted into some geometrical or mathematical form, as discussed in Chapter 1. For example, a fire tower might be represented by a point, a stream by a set of connected straight lines, and a lake by a polygon boundary.