At the beginning of the assignment you can re-use existing connections instead of a complete connection search calculation. This will significantly speed-up the assignment and allows for subsequent usage of external tools for the connection search.
The import can use data from various sources:
- from a connection file
- from an existing assignment result in the network
Once the import is finished the assignment will continue in the same way as after the internal connection search. The source especially does not have an impact on the connection choice (Connection choice).
Imported connections have preset times of arrival and departure which depend on the assignment time period that was set for their calculation. For the import, you can modify the assignment time period. In this case the temporal distance of the connections from the new assignment time period is ∆T in the impedance as usual. In general this ensures, that there is almost no demand for connections which are far outside. And they do not have a significant impact on the skim calculation. Please note that the skim service of frequency, however, always takes into consideration the absolute number of connections – not taking into consideration their temporal position. For this skim, identical assignment time periods are recommended for the export and import of connections.
Import from an existing assignment result
The usage of paths which were calculated in a previous assignment is similar to the PrT assignment option Use current assignment result as initial solution (Using Visum: Parameters of Equilibrium assignment). However, unlike PrT, the path search may be dropped completely for public transport. You need to enter a demand segment for which an assignment has already been calculated.
The existing assignment needs to satisfy the following recommendations, otherwise the paths resulting from the assignment cannot be re-used:
- A timetable-based assignment is required.
- For the calculation, the option 'Save paths as connections' needs to be selected. Please note that this setting differs from the default (Using Visum: Settings for saving PuT assignment results).
- The demand segment has to belong to the same mode as the demand segments which are currently to be assigned.
Import from a connection file
Alternatively, you can read connections from a file.
For this purpose, use the same connection files as for the connection import procedure (Use case (1): External choice / Connection import). In contrast to this procedure, the import from a connection file ignores the stored volumes, only the paths are read from file.
Connection files can be applied to any demand segment. Make sure that the paths from the connection file and the mode of the currently to be assigned demand segments match.
The import automatically identifies the level of the stored fare information (File format for connection import and export). Please note that the current option setting Save imported fare data (Using Visum: Settings for saving PuT assignment results) determines whether and how these fares are imported.
