13.2 Model and Data Structure

a. Requirement of the Model and Data Structure
In order to process and manage geographic information by computers, it is necessary to describe the spatial location and distribution, as well as the attributes and characteristics, according to a specified form, termed a spatial representation model with a standardized data structure.

b. Modeling and Data Structure
Geographic information can be represented with geometric information such as location, shape and distribution, and attribute information such as characteristics and nature, as shown in Figure 13.2.1.

Vector and raster forms are the major representation models for geometric information.

(1) Vector form and its data structure
Most objects on a map can be represented as a combination of a point (or node), edge (or arc) and area (or polygon). The vector form is provided by the above geometric factors. The attributes are assigned to points, edges and areas.

The data structure is specified for the vector form as follows.

A point is represented by geographic coordinates. An edge is represented by a series of line segments with a start point and an end point. A polygon is defined as the sequential edges of a boundary. The inter-relationship between points, edges and areas is called a topological relationship. Any change in a point, edge or area will influence other factors through the topological relationship. Therefore the data structure should be specified to fulfill the relationship, as for the example as shown in Figure 13.2.2.

(2) Raster form and its data structure
In the raster form, the object space is divided into a group of regularly spaced grids (sometimes called pixels) to which the attributes are assigned. The raster form is basically identical to the data format of remote sensing data.

As the grids are generated regularly, the coordinates correspond to the pixel number and line number, which is usually represented in a matrix form as shown in Figure 13.2.3.


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