Geographic Information Systems: An Introduction, 3rd Edition

3.6 Extension of the Traditional GIS Data Model

3.6.1 Overlapping polygons

In the real world, one specific area or field may have many different characteristics; one area may in reality represent a number of entities or object types, such as coniferous forest, protected area, Property No. 44/133, and so on. We experience, on a daily basis, that it is the area as an entity that carries the information. However, in our real-world model we split phenomena into entities (entity: a real-world phenomenon that is not divisible into phenomena of the same kind) and allow the entities to be bearers of information. This model will allow an entity to represent only one phenomenon (e.g., only coniferous forest or only protected area). To adapt the model to reality, overlapping phenomena (entities/objects) are separated into different layers (Figure 3.11).

Reality is thus adapted to fit into a layer system, which is also traditionally used in map presentation. Because this type of model does not have overlapping polygons, each polygon in the different layers may be represented with one single set of coordinates, such as the center of gravity of the polygon. The polygons' attributes can thus be connected to this point. This is a very practical solution in regard to data management; thus many GISs make use of it. In the real world, areas are not divided into any form of horizontal two-dimensional physical layers. However, modern GISs have developed data structures that can handle overlapping polygons, ...