## DESCRIPTION

v.net.distance finds the nearast element in set to for every point in set from.

## NOTES

These two sets are given by the respective layer, where and cats parameters. The type of to features is specified by to_type parameter. All from features are points. A table is linked to output map containing various information about the relation. More specifically, the table has three columns: cat, tcat and dist storing category of each from feature, category of the nearest to feature and the distance between them respectively. Furthemore, output map contains the shorest path between each cat, tcat pair. Each path consist of several lines. If a line is on the shorest path from a point then the category of this point is assigned to the line. Note that every line may contain more than one category value since a single line may be on the shortest path for more than one from feature. And so the shortest paths can be easily obtained by querying lines with corresponding category number.

The costs of arcs in forward and backward direction are specified by afcolumn and abcolumn columns respectively. If abcolumn is not given, the same cost is used in both directions.

v.net.distance will not work if you are trying to find the nearest neighbors within a group of nodes, i.e. where to and from are the same set of nodes, as the closest node will be the node itself and the result will be zero-length paths. In order to find nearest neighbors within a group of nodes, you can either loop through each node as to and all other nodes as from or create a complete distance matrix with v.net.allpairs and select the lowest non-zero distance for each node.

## EXAMPLES

Find shortest path and distance from every school to the nearest hospital and show all paths.

Streets are grey lines, schools are green circles, hospitals are red crosses, shortest paths are blue lines:

    # connect schools to streets as layer 2
    v.net input=streets_wake points=schools_wake output=streets_net1 operation=connect thresh=400 alayer=1 nlayer=2

    # connect hospitals to streets as layer 3
    v.net input=streets_net1 points=hospitals output=streets_net2 operation=connect thresh=400 alayer=1 nlayer=3

    # inspect the result
    v.category in=streets_net2 op=report

    # shortest paths from schools (points in layer 2) to nearest hospitals (points in layer 3)
    v.net.distance in=streets_net2 out=schools_to_hospitals flayer=2 tlayer=3

## SEE ALSO

v.net.path, v.net.allpairs, v.net.distance, v.net.alloc

## AUTHORS

Daniel Bundala, Google Summer of Code 2009, Student
Wolf Bergenheim, Mentor
Markus Metz

Last changed: $Date$