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Identification of Classes

The Major Classes in the RNDM are divided into base classes and concrete classes. A base class is an abstract class that is used to represent a template for a geographic object. It can't be instantiated. Typically, there will be subclasses of it which will inherit it's properties down the hierarchy. A concrete class, on the other hand, can be instantiated with real world objects.
The base class is: GeoObject. GeoObjects is a template for the geographic objects in the model (See [1] for details).

Analysis of the requirement specifications of the shortest path problem shows that most of the queries pertaining to the RNDM would be asked with reference to certain well understood geographic objects in the model, such as country, prefecture, or city. Prefecture, and city etc., are the container geo-objects that contain other container and non-container geo-objects. Roads and rivers are the examples of non-container geo-objects. Thus the geographic objects in this application can be broadly divided into container geo-objects which contain other geographic objects and non-container geo-objects which don't contain any other geo-object. Due to this fact, the following two abstract classes are defined:
ContainerGeoObject class is defined to represent container geo-objects in the model. In order to add real life container geographic objects like country, prefecture, and city in the model |mboxconcrete subclasses Country, Prefecture, City, and Neighbourhood of the subclass ContainerGeoObject are defined.
NonContainerGeoObject class is defined to represent the non-container objects in the model.
The class Road is an abstract subclass of the class NonContainerGeoObject that is used as a template for road objects in the model. In order to add real life road objects in the model concrete subclasses, Expressway, CityExpressway, NationalRoad, PrefectureRoad, CityRoad, and MinorRoad of the class Road have been defined. We also define a concrete subclass RoadSegment of the class Road. To add real life road segment we define three subclasses PrefectureSegment, CitySegment, and NeighbourhoodSegment of the class RoadSegment. The class hierarchy is shown in Figure 2.
The subclass Node represents cross point of two roads, start/end point of a road, or cross point of boundary of a geographic object and road.
The subclass Link represents the part of the road from node to node in this model. To represent this a class Link has been defined. Link in the road network data model may be a ramp, a bridge or a tunnel.

Figure 2: Class hierarchy
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\epsfile{file=classheirarchy4.eps,vscale=1,hscale=1}
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These geographic objects are arranged in a containment hierarchy with country containing prefectures, expressways, city expressways and national roads etc. And prefecture containing cities, prefecture roads, and prefecture segments of expressways, city expressways, and national roads. Similarly it can be explained down the heirarchy as shown in Figure 3.
Due to the complexity of showing every thing in one figure we used the word group. The group at country level means the group of the prefecture segments of expressways, city expressways, and national roads which make a road at country level. The group at prefecture level means the group of city segments of each road that makes a prefecture road or a prefecture segment of a road and so on. At the lowest level the group consists of the links that make the neighbourhood road or neighbourhood segment of any road.

Figure 3: Containment hierarchy
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At this stage in the design process it is noticed that non-container geo-objects can be divided into composite segments. For example, non-container geo-object such as, expressway is composed of prefecture segments of the expressway which are contained entirely by prefectures. And similarly, prefecture segments and prefecture roads are composed of city segments which are contained entirely by cities as shown in Figure 4.

Figure 4: Composite hierarchy of Road
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\epsfile{file=compositeheirarchy2.eps,vscale=1,hscale=1}
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next up previous
Next: Significance of the Model's Up: DESIGNING THE ROAD NETWORK Previous: DESIGNING THE ROAD NETWORK
M.Abaidullah ANWAR
1999-11-25