GIS and Transportation Modeling
Published in: PLANetworks, Issue 1, Spring 1997, p. 5.
There is a powerful synergy between GIS and transportation modeling systems. While most GIS systems cannot do the complex network simulation and analysis that transportation-specific packages such as EMME/2 and TRANPLAN can, GIS excels at editing, displaying, and plotting networks and related data. By using GIS with a transportation modeling package, it is possible to extend modeling capabilities and open up new possibilities for developing, using, and maintaining transportation data.
ArcInfo can be used to store, edit, display, and plot a simulation network, both before assignment and after. Before a simulation is run, ArcInfo can make maintaining simulation networks easier in many ways. Using software such as the ArcTools, ArcInfo can manage a whole range of simulation networks by maintaining a base network and tracking projects that will change that network over time. By maintaining one network rather than many, problems of network tracking, documentation, and consistency are eliminated. Also, ArcInfo provides a range of powerful network editing tools that are simply unavailable in most modeling packages.
ArcInfo also provides a powerful way to display model results. Because you are not limited to using straight-line approximations of street geometry, it is possible to combine modeling data with existing GIS data and annotation utilizing ArcInfo's rich set of cartographic tools. Also, it is possible to conduct further analysis of transportation data in ArcInfo using the Network and GRID modules. For example, you could develop a comprehensive urban air quality model combining both stationary and mobile emission sources to highlight areas of potential violation of air-quality standards.
Because of the complimentary strengths of transportation models and GIS systems, it is only logical to use them together to create a powerful set of modeling tools. Once established, such a system can simplify network maintenance and reduce errors while at the same time extending both the analytical and display capabilities of existing travel forecasting models.
Several state Departments of Transportation and MPOs have begun to utilize ArcInfo and ArcView with transportation models. In some cases the GIS is simply used to display the model results with other GIS datasets as a backdrop. In more sophisticated applications, such as the Southern California Association of Governments, ArcInfo is used to update and manage the network as well as display the model results. In SCAG, the model networks are integrated with the Thomas Brothers street centerline files so that assigned flows are displayed on real streets. This helps the modelers to validate model output and identify local inconsistencies, caused by network coding errors or the wrong location of zone centroid connectors.
GIS can also play a useful role in organizing inputs to other stages in the travel demand forecasting process. Examples include compiling data in Traffic Analysis Zones for trip generation, formatting the zonal data into trip tables for trip distribution modeling and organizing mode choice data by geographic area as part of mode split analyses.
ArcInfo does not provide off-the-shelf application tools for transportation modeling and most applications require the user to develop their own application bridges between the GIS and the simulation model. Most users have found ArcInfo 7.1 the best environment to develop these tools with ArcView as a GUI to display the results with other data for non-GIS specialist staff.
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