Geographic Information Systems: An Introduction, 3rd Edition

5.4 Models for Movable Objects

A considerable part of the real world consists of movable objects: vehicles on a road network that carry passengers or goods, or water running over the terrain surface. One should also be able to realize this aspect of the real world in GIS. Special models have therefore been developed to handle these conditions.

5.4.1 Network model

The network model comprises road systems, power grids, water supply and sewerage systems, and the like, all of which transport movable resources. The most usual type of networks for which GIS models are developed are road systems; the following description therefore refers to road systems. For most purposes, reality can be simplified to a model that can handle two different situations:

1. Displacement of resources or objects from one place to another
2. Allocation of resources or objects from or to a center

As with other GIS data models, this model is based on geometry and attributes (Figure 5.16). The geometry in the network is represented by lines consisting of connected lines of vector data. The geometry of a road system is represented by the center line of the road. This model assumes topological data built up of links and nodes. Every link and every node in a network must have a unique identity. The attributes are tied to links and nodes in a linear system through these IDs and are intended to describe the total accessibility of the system. We can thus state that the model is based on three basic relationships:

1. Continuous, ...