Skim matrices describe properties of each relation from an origin zone A to a destination zone B in the traffic model. Each individual skim (for example the travel time in a vehicle) is extracted from the path properties from A to B, which belong to a demand segment. The skim data is then aggregated with the relative share of demand, which the path would attract, to a skim value for the OD pair. This also applies if there is no demand for the relation from A to B, because distribution does not depend on the demand.
The calculation of skim matrices differs between PrT and PuT on some points. The calculation of PrT skim matrices is either based on present paths from a previously calculated assignment, or for each OD pair the optimum path with regard to the impedance is determined (in the possibly loaded network). Compared to an assignment, the network is not loaded in this case. Because in this case there is only one path per relation, the skim value is extracted directly from this path. If, however, paths from an assignment are used for skim matrix calculation, the value of the minimum or maximum path impedance can be output as skim value, or the weighted or unweighted mean value calculated from all paths by OD pair.
In PuT always more than one route or connection is calculated per OD pair, and the skim value is derived from these. In addition to the average determination, optionally weighted with the demand share, the output of properties of the path with the least perceived journey time (PJT, timetable-based procedure) or with the least impedance (headway-based procedure) as well as quantiles are available as additional aggregate functions. The skim is especially directly dependent on the applied search strategy. Because not only the saved, but all paths found are included in skim matrix calculation, the result differs from the result subsequently derived from the paths. This is the case if the demand becomes zero on some paths by an explicitly requested rounding and the path is therefore not saved but used for skim matrix calculation. If demand and volume rounding is switched off, such differences cannot occur.
