## NAME

v.net.path

## DESCRIPTION

v.net.path can find shortest path(s) on the vector network. Costs may be either line lengths, or attributes saved in a database table. These attributes values are taken as costs of whole segments. If read from the table, arcs' costs may be different in both directions. Shortest paths are written to output vector map and attached attribute table.

Nodes can be piped into the program from file or from stdin. The syntax is as follows:

    id start_point_category end_point_category

or

    id start_point_x start_point_y end_point_x end_point_y

Points specified by category must be exactly on network nodes, while, when specifying coordinates, the next node to a given coordinate pair is used.

Attribute table will contain following attributes:

- cat - path unique category assigned by module
- id - path id (read from input)
- fcat - from point category
- tcat - to point category
- sp - result status:
  - 0 - OK, path found
  - 1 - node is not reachable
  - 2 - point of given category does not exist
- cost - travelling costs (on the network, not to/from network)
- fdist - the distance from first point to the network
- tdist - the distance from the network to second point

## NOTE

If the columns 'afcol', 'abcol' and 'ncol' costs are not specified, the length of network segments is measured and zero costs are assumed for nodes.

When using attributes, the length of segments is not used. To get more precise results, length should be taken indirectly into account by attributes. For example, to get the fastest path the columns 'max_speed' and 'length' are required. The correct fastest path can then be found by specifying `afcol=length/max_speed` (pg driver). If needed, the line length can be calculated and written to the attributes table by v.to.db.

## EXAMPLE

Shortest path from two digitized nodes (Spearfish):

    g.copy vect=roads,myroads
    v.db.addcol myroads col="forward double precision, backward double precision"

    # define traveling costs as inverse of speed limit:
    v.db.update myroads col=forward val=1/50
    v.db.update myroads col=backward val=1/50
    v.db.update myroads col=forward val=1/75 where="label='interstate'"
    v.db.update myroads col=backward val=1/75 where="label='interstate'"
    v.db.update myroads col=forward val=1/5 where="label='unimproved road'"
    v.db.update myroads col=backward val=1/5 where="label='unimproved road'"
    v.db.update myroads col=forward val=1/25 where="label='light-duty road, improved surface'"
    v.db.update myroads col=backward val=1/25 where="label='light-duty road, improved surface'"
    v.db.select myroads
    echo "1|601653.5|4922869.2|start
    2|593330.8|4924096.6|end" | v.in.ascii cat=1 x=2 y=3 out=startend col="cat integer, \
                             east double precision, north double precision, label varchar(43)"

    v.db.select startend

    #create lines map connecting points to network (on layer 2)
    v.net myroads points=startend out=myroads_net op=connect thresh=200
    g.region vect=myroads_net
    d.mon x0
    d.vect myroads_net
    d.vect startend col=red

    v.db.select myroads_net

    d.vect myroads_net icon=basic/triangle fcol=green size=12 layer=2
    d.vect myroads_net disp=cat type=point lsize=14 layer=2
    # ... the 'start' and 'end' nodes have category number 1 and 2

    # ID as first number, then cat1 and cat2
    echo "1 1 2" | v.net.path myroads_net afcol=forward abcol=backward out=mypath
    d.vect mypath col=red width=2

## SEE ALSO

d.path, v.net, v.net.alloc, v.net.iso, v.net.salesman, v.net.steiner, v.to.db

## AUTHOR

Radim Blazek, ITC-Irst, Trento, Italy
Documentation: Markus Neteler

Last changed: $Date: 2007-08-03 14:28:13 +0200 (Fri, 03 Aug 2007) $

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