Research Methodology
This research first investigated real-world features and their spatial and aspatial properties. The geometry and topology were treated as a property of the feature. The network connectivity model was designed including the network connectivity rule. A relational database structure was designed to fit the conceptual model and implemented to collect the data come from various dataset. The application was implemented using the Java programming language, and JDBC [20] was used for connecting to databases. Several tools were created to support the application, especially for network analysis. The tool for building topology was created and tested to build topology based on the connectivity rules. The network analysis functions - network tracing and shortest path – were developed that incorporated analysis of directed networks.
Data Model
To understand the real world, the characteristic of real world features must be studied and modelled. In the real world features are described by some descriptive terms, by physical location and by their ability to connect both physically and logically to other features. Thus a “road feature” could be described by aspatial attributes should as name and length, by a geometry representing its physical location, by topology to represent how it is physically connected to other features and by join relationships to represent logical connections with other features. The modelling of attributes, geometry and relationships are standard fare for most GIS and many database systems, so the following section concentrates on the modelling of rule based topological structures to facilitate network analysis functions.
The real world feature
Phenomena in the real world can be modelled as real world features. The real world is composed of many kinds of real-world features. The characteristics of a feature can be represented by its properties. There are four kinds of feature properties: physical, relational, geometrical, and topological.
A
physical property is usually alphanumeric data stored as a number or
text. Relationship properties are used for representing the logical
relationship between features. The shape of a feature is represented
by a geometry. For a 2D coordinate system there are three basic
types of geometry: point (x, y pair), chain (series of connected x,
y pairs) and area (series of connecting x, y pairs making up one or
more complete rings). Topology is commonly used to describe the
physical connectivity between features.
Figure 2 Real world phenomena
Network Connectivity
- Topology
Topology is the common term used to describe
physical connectivity between features. Topology is generally
represented by links and nodes. A feature instance is connected to
another feature instance via a connection point. This connection
point is described by a node, and the path between two nodes is
described by a link. Topology is derived from the underlying
geometry.
Figure 3. Link and Node model
There are two common properties for the link: cost and direction. Cost is the value which is taken into account to find the best path. Commonly the cost is the distance of the link which is adequate for most simple network analysis problems. Direction is used for specifying which direction the network can travel on that link. There are also two properties for the node: in/out cost and degree. In/out cost is the accumulated distance from the starting point that used to find the next distance value at another node of the same link. Node degree presents the number of links associated with it.
- Directional network
For some applications topological features require direction as well as connection. If we consider the flow of water in a river, the topology must be modelled to take into account the flow direction of the water. However for other applications such as analysing boat traffic on a river it is more sensible to model the network as non directional or two-way. Moreover in a road network, if we consider the road feature, it may be one way or two way and as is the case in some cities it may change depending on the time of day etc.. Thus there are requirements to be able to model the direction of connectivity whilst retaining flexibility to suit the application in question.
There are several ways to handle the directional flow of a network. Some systems use a special feature to set the directional flow of the link, whereas other systems set the directional flow in the application using additional coding. This research sets the directional flow as a property of a line and provides the database structure for the directional network as a directed line. A Link feature derived from the directed line is a directed link.
- Connectivity types
In order to model real world complexity we also need to be able to express the concept of different types of connectivity. Whilst it may be acceptable to allow road features to connect if they share the same 2D space, it is not appropriate for all situations e.g. fibre optic cables, water mains etc…
To enable the different types of feature
connectivity, we need to model the three ways to connect two link
features: end-connection, middle-connection and cross-connection,
and the two ways connecting link features to node features:
end-connection and middle-connection.
Figure 4 Connectivity types
To build an accurate
model of the real world situation the type of connectivity between
feature types must also be modelled successfully.