Basically, travelling for a long distance trip, we need the small scale map for planning the travel route. The route may pass many cities. We need larger scale map to find an optimal route of the cities. The small scale network is analyzed first to get the route. The inbound and outbound roads of each city are identified while analyzing. Then the interesting city network analysis is performed using the same inbound and outbound road as origin and destination respectively. Together with the dynamic network analysis and road’s variable traffic speed, we simulate the real situation of our route.
In Thailand, the road sign is not as good as in the developed country. When we want to travel between 2 cities, we can use GIS network analysis software to find the best route. However, the selected routes will pass several cities and towns. The problems will start when you enter towns and cities which do not have good road signs. The result is you may loose the orientation or direction in them.
Our solution is to do 2 levels network analysis, the first level is country road network, the second level is cities road network. The linkage between these 2 road networks are created. When the first level analysis is done. Users can select the town or city which are on the analyzed route, to do additional network analysis in city level. The city road network analysis is automatically handled by software.
The software is written by Visual Basic. GIS component is MapObjects. The Network Analysis module is developed by using Dijkstra’s Algorithm.
Two levels of network layers must be considered here including country level network model and city level network model. However, the network data model of these 2 levels are identical. The details of each level layers and network data model are as followings:
Each node of this level of network can be either an intersection or the city. These types of node must be specified in the attribute table. In case of the city, the city code must be entered. The database structure is illustrated in figure 1.
Figure 1. Database Structure of Country level network layers
In this level of network, the node layer that represents all intersections in the city contains only the unique ID. The figure 2 illustrates database structure of these 2 layers.
The name of the layers are defined as CityCode_road and CityCode_int for street and intersection layer respectively.
Figure 2. Database Structure of City level network layers
Moreover the unique ID of the country level network is recorded in the inbound and outbound roads of the city level. The application recognizes the inbound road and outbound road of the selected city in the country level network. This recognition is useful for automatically performing the network analysis in the city level by using the same inbound and outbound roads.
The figure 3 illustrates the linkage between country and city level networks.
Figure 3. Linkage from country level network to city level network
The origin node must be selected first. The departure time is then specified. The network is analyzed by using Dijkstra’s Algorithm together with the travel and turning costs of the arrival time at each road and intersection. This analysis creates the data structure to keep the optimal path from an origin to each intersection and the total cost of travel from origin to each intersection. After selecting the destination node, the optimal path is created and travel cost and travel time are derived.
Both country and city level networks use this network data model when building and analyzing the network. The only difference is that an origin node, destination node, and departure time of city level network are derived automatically from the analysis of country level network. The departure time of city level path is the arrival time of the city node.
The application software comprises of 3 main modules. These modules are the Network Topology, Network Analysis module and Network Editor module. Both levels of network can be processed by using these 3 modules.
The Network Topology module is used for building Arc-Node topology and creating node layer. Then the network topology is loaded into memory for network analysis. The default edge travel cost can be loaded by using this module.
The purpose of Network Analysis module is for selecting the origin node, specifying the departure time, analyzing the network, selecting the destination node, and creating the optimal route. After having the optimal route, the city node is selected in order to load the city network layers, derive necessary information , and calculate the optimal path for the city network.
Network Editor module includes 3 submodules. These submodules are Network creation, Linkage of networks creation, and Network parameter editor.
In our research, both levels of network were already created in ArcInfo. The ArcInfo coverages are then converted into Shapefile format.
To create the linkage of networks, ArcView GIS is used to get the unique ID of all inbound and outbound road of each city node. Then the highway field value of inbound and outbound road of city level network is filled in.
Basically, the network parameters include the traffic speed along the road and turning cost. The network parameter editor lets user edit the traffic speed and turning cost in both levels of network.
Figure 4. Country level road network
Figure 5. Input traffic speed
Figure 6. Input turning waiting time
Figure 7. Select origin city (big yellow node) and analyze network with departure time of 9.00
Figure 8. Select destination city ( big blue node) and analyze the shortest traveling time path
Figure 9. Result of city level analysis of selected city node.(Departure time, inbound road, and outbound road are derived from country network)
Figure 10. Adjust the traffic speed of the city network
Figure 11. Result of adjusting traffic speed in the city network
Figure 12. Adjust turning waiting time of an intersection in city network
Figure 13. Result of adjusting turning waiting time in the city network
The integration of 2 scales road network analysis is practical. Travelers can make an optimal route plan in a small scale country map. Then they further their analysis within a large scale city map in order to understand the street pattern and analyze an optimal path from inbound road to outbound road. With the Network Parameters Editor tools, travelers simulates the virtual road networks of country and city levels by updating traffic speed and turning cost. However collecting the traffic speed and turning waiting time of each hour for all roads and intersection is quite difficult. Another problem is defining a linkage between 2 levels of network. Typically, almost all of city level maps in Thailand have a poor quality in term of orientation, measurement, and accuracy so that it takes time to find the identical roads in a country map and city map. If the traffic speed, turning waiting time, and linkage of networks are completed, the combination of two network models are an efficient and practical model of the real world road network.
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