Geographic Information Systems: An Introduction, 3rd Edition

4.2 Vector Data Model

The basis of the vector model is the assumption that the real world can be divided into clearly defined elements, each of which consists of an identifiable object with its own geometry of points, lines, or areas (Figure 4.1). In principle, every point on a map and every point in the terrain it represents is uniquely located using two or three numbers in a coordinate system, such as in the northing, easting, and elevation Cartesian coordinate system. On maps, coordinate systems are commonly displayed in grids with location numbers along the map edges (Figure 4.2). On the ground, coordinate systems are imaginary, yet marked out by survey control stations. Data usually may be transformed from one coordinate system to another. (See Chapter 6 for further details on coordinate systems.) Points, lines, and areas are mathematical figures that are used to describe the positions and extensions of geographical objects.

Points

Points are the fundamental and simplest form of geographical objects and are zero-dimensional because they have no extension. Each point is represented by a coordinate pair. As discussed later, we also operate in a GIS with a special type of point called a node (Figure 4.3). A point has no boundary.

Figure 4.1

In the vector data model, the objects are described as points, lines, or areas (polygons). These three geometric phenomena may be described ...