This exercise allows you to experiment with the GRASS 6.4 tools for network analysis.
The usual questions you may ask about a network (a collection of connected lines and nodes) are the following:
- what is the shortest path between two points (nodes?)
- can you compute equidistant zones along the network given some nodes?
- can you divide the network into sub-networks of nearest elements to given nodes?
- what is the minimum network connecting a set of given nodes (Steiner trees)?
- what is the shortest route passing from all the given points only once (travelling salesman problem)?
In the real world the above questions should be asked in terms of
costs of passing through links and nodes. For instance, in terms of travelling time, the same distance can have different costs according to road type; or in a water facility system it can be affected by pipes diameter or leakages. In this tutorial you can try to use the network analysis tools with various cost attributes along the network.
The aim of the exercise is to set up a GIS network to support the fire brigade interventions in the area of North Carolina. Here follows the steps of this process and the tools used:
- set up of network analysis: geometry and the appropriate database tables will be connected (v.net);
- search for the shortest path between fire stations and the road network in order to prepare a displacement map. Computation of the shortest path between some fire stations (v.net.path);
- zone the area according to the distance from fire stations to create a map of forecasted intervention time (v.net.iso);
- allocate across the network each fire station to a competence area according to the distance from the roads (v.net.alloc);
- find the shortest subnetwork linking all the fire stations in order to connect them with a high speed cable running along the streets.
- find the shortest path connecting all the fire stations in order to optimise the route of suppliers and inspectors (v.net.salesman).
You need to do the set up of the data first, then the different modules can be done in any order.