Costs may be either line lengths, or attributes saved in a database table. These attribute values are taken as costs of whole segments, not as costs to traverse a length unit (e.g. meter) of the segment. For example, if the speed limit is 100 km / h, the cost to traverse a 10 km long road segment must be calculated as length / speed = 10 km / (100 km/h) = 0.1 h. Supported are cost assignments for arcs, and also different costs for both directions of a vector line. For areas, costs will be calculated along boundary lines.
The input vector needs to be prepared with v.net operation=connect in order to connect points representing center nodes to the network.
Points specified by category must be exactly on network nodes, and the input vector map needs to be prepared with v.net operation=connect.
v.net.salesman.opt uses the same heuristics like v.net.salesman.opt to find a good tour which is not necessarily the optimal tour. The tour can be optimized with two methods: bootstrapping and genetic algorithm (GA). The bootstrapping method removes a subtour from the tour and reinserts the nodes resulting in a shorter tour. This is applied on intermediate tours and the final tour.
The genetic algorithm first creates several initial tours. From these tours nearly identical toursare eliminated. The remaining tours are recombined to create new, better tours. All tours are now mutated. The sequence of Selection, Recombination, Mutation is repeated until there is no better solution. Finally, the best tour is optimized with bootstrapping.
# North Carolina # prepare network by connecting schools to streets v.net -c input=streets_wake points=schools_wake output=streets_schools \ operation=connect alayer=1 nlayer=2 thresh=1000 # verify data preparation v.category in=streets_schools op=report # type count min max # point 6 1 6 # find the shortest path v.net.salesman.opt in=streets_schools ccats=1-167 out=schools_tour # Resulting tour length: 551551.662 # find the shortest path, optimize with genetic algorithm v.net.salesman.opt in=streets_schools ccats=1-167 out=schools_tour_ga method=ga # Resulting tour length: 521191.083 # The original tour was 5.8% longer # find the shortest path, optimize with bootstrapping v.net.salesman.opt in=streets_schools ccats=1-167 out=schools_tour_bs method=bs # Resulting tour length: 510693.965 # The original tour was 8.0% longer
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