Each record in a relational database references something: a person, object, idea, equation, feature, subject—some unique entity. In a GIS, the entity is usually a spatial three-dimensional feature such as a parking meter, lake, railroad, and so on.25 These 3-D features are almost always reduced to abstract objects of fewer dimensions. The parking meter is a point, the railroad is a set of lines, and the lake is an area bounded by a sequence of lines.
Usually, a record in a relational database references a person or object without respect to its current location. A subject of a record in a relational database frequently moves around—like cars or people. However, when the subjects of a relational database are fixed in space, the position, or positions, of the feature may be included in the description of the feature along with the attributes. In one sense, the location of the object becomes one of its attributes.
Here, for example, is a map and part of a relational database that together describe fire hydrants in a town. (See Figure 1-13.) Each hydrant has a unique number and is, thus, the key field. Fire hydrants (each of which has a latitude and longitude specification) correspond one-to-one with the records in the database.
Let’s consider another definition of ...