visual transport modeller: accessibility model sequence
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Accessibility Modelling may be done for a Highway network, a Public Transport network, or a network that combines both. A Public Transport network may be supplied in the form of CIF files that define the routes and all the services that run on them and Visual Transport Modeller provides the means to convert these into a usable network in the standard Visual Transport Modeller format. This is done on the CIF to Visual-TM Converter tab on the Accessibility form.
Although CIF files store the routes and journeys in a fairly standard format, there are different ways of storing the locations of the bus stops and the routes may either be in one large file or many smaller ones. The software has been written to handle the variations discovered so far.
Adding zone connectors
The Public Transport network obtained from the CIF files has only the routes and their links and bus stops. The next step is to use the Find Zone Connectors tab to add the zone centroids and zone connectors to the network from a spreadsheet that lists the zone numbers along with the centroid coordinates. The other input file is the network file containing the routes.
Note that if a network already exists it is not necessary to produce a new one using CIF files. The Ashtest project has an existing network which we will use to demonstrate the rest of the accessibility functionality.
Obtaining accessibility results
The first step in obtaining accessibility results is to produce a Standard Set of Skims. This pbSkim.dat file is one of the main inputs for finding the accessibility.
Accessibility results are obtained on the Zone Accessibility from Skim tab. The skim file is selected as the first input and the Excel file containing the zone data is selected as the Output Area file. The data can be at ward level or any other zone system, but the data in the file needs to conform to the zones in the network and the skim. The range with the data is chosen and then the columns with the zone numbers and the population or number of households in the zones.
The OA Destinations of Interest file lists the zone numbers to which the accessibility is required e.g. every zone that contains a hospital. The zone numbers should be in a range with a single column or in the first column of a range that has multiple columns. An output file name should be entered and a Threshold time entered, which is in minutes. If the travelling time from a zone to any of the Destinations of Interest zones is below this threshold, then the population or households within the zone are deemed to meet the accessibility conditions.
A typical example is shown below:
Click Go to perform the calculation. The percentage of households or population that meet the accessibility condition will be displayed on the bottom of the form at the end. The output file is a Comma Separated Variable file that lists each zone number followed by its population and then the minimum time to get from it to one of the Destinations of Interest. If a zone has a value of a million in this field, it means that the path building was unable to find a route from that zone through the network to any of the destinations of interest.
Plotting accessibility results
Once the output csv file has been produced, the results can be plotted on the Network display. On the Plotting Options tab of the Accessibility form the user can choose the ranges and colours for plotting.
Clicking the plot button will colour the zone centroids on the Network display.
Zoom in for more detail.
To remove the Accessibility plot, right click the network with the Panning option unticked.
Exporting results
Accessibility results can be displayed in a GIS package. A .mid file can be produced with the accessibility model run, and this should be given the same name as the zone system .mif file. This can then be imported into MapInfo and a thematic map created, showing the journey times from each zone to the nearest hospital, school, or other place of interest.
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