Ubuntu Manpage: v.net.path - Finds shortest path on vector network.

NAME

       v.net.path  - Finds shortest path on vector network.

KEYWORDS

       vector, network, shortest path

SYNOPSIS

       v.net.path
       v.net.path help
       v.net.path  [-gs]  input=name  output=name   [type=string[,string,...]]   [alayer=integer]
       [nlayer=integer]   [file=name]   [afcolumn=string]   [abcolumn=string]    [ncolumn=string]
       [dmax=float]   [--overwrite]  [--verbose]  [--quiet]

   Flags:
       -g
           Use geodesic calculation for longitude-latitude locations

       -s
           Write output as original input segments, not each path as one line.

       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Name of input vector map

       output=name
           Name for output vector map

       type=string[,string,...]
           Arc type
           Options: line,boundary
           Default: line,boundary

       alayer=integer
           Layer number
           Arc layer
           Default: 1

       nlayer=integer
           Layer number
           Node layer
           Default: 2

       file=name
           Name of file containing start and end points. If not given, read from stdin

       afcolumn=string
           Arc forward/both direction(s) cost column

       abcolumn=string
           Arc backward direction cost column

       ncolumn=string
           Node cost column

       dmax=float
           Maximum distance to the network
           If  start/end  are given as coordinates. If start/end point is outside this threshold,
           the path is not found and error message is printed. To speed up the process, keep this
           value as low as possible.
           Default: 1000

DESCRIPTION

v.net.path determines least costly, e.g. shortest or fastest path(s) on a vector network.

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 both arcs and nodes, 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.

Nodes and arcs can be closed using cost = -1.

Least cost paths are written to the output vector map with an attached attribute table.

Nodes can be

piped into the program from file or from stdin, or

defined in the graphical user interface ("enter values interactively").
The syntax is as follows:
id start_point_category end_point_category
(Example: 1 1 2)

or
id start_point_x start_point_y end_point_x end_point_y

Points specified by category must be exactly on network nodes, and the input vector map
needs to be prepared with v.net operation=connect.

When specifying coordinates, the next network node to a given coordinate pair is used.

The attribute table will contain the 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

NOTES

       Nodes and arcs can be closed using cost = -1.

       If  the  cost columns 'afcol', 'abcol' and 'ncol' 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  accurate  results,  the
       line length must be taken into account when assigning costs as 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. If not yet existing,
       the column containing the line length ("length") has to  added  to  the  attributes  table
       using v.to.db.

EXAMPLE

       Shortest (red) and fastest (blue) path between two digitized nodes (Spearfish):

       # Spearfish
       echo "1|601955.1|4916944.9|start
       2|594385.6|4921565.2|end" | v.in.ascii in=- cat=1 x=2 y=3 out=startend col="cat integer, \
                                east double precision, north double precision, label varchar(6)"
       v.db.select startend
       g.copy vect=roads,myroads
       # create lines map connecting points to network
       v.net myroads points=startend out=myroads_net op=connect thresh=500 alayer=1 nlayer=2
       # set up costs
       # create unique categories for each road in layer 3
       v.category in=myroads_net out=myroads_net_time opt=add cat=1 layer=3 type=line
       # add new table for layer 3
       v.db.addtable  myroads_net_time  layer=3  col="cat integer,label varchar(43),length double
       precision,speed double precision,cost double precision,bcost double precision"
       # copy road type to layer 3
       v.to.db myroads_net_time layer=3 qlayer=1 opt=query qcolumn=label columns=label
       # upload road length in miles
       v.to.db myroads_net_time layer=3 type=line option=length col=length unit=miles
       # set speed limits in miles / hour
       v.db.update myroads_net_time layer=3 col=speed val="5.0"
       v.db.update myroads_net_time layer=3 col=speed val="75.0" where="label='interstate'"
       v.db.update myroads_net_time layer=3 col=speed val="75.0"  where="label='primary  highway,
       hard surface'"
       v.db.update myroads_net_time layer=3 col=speed val="50.0" where="label='secondary highway,
       hard surface'"
       v.db.update myroads_net_time layer=3 col=speed val="25.0"  where="label='light-duty  road,
       improved surface'"
       v.db.update myroads_net_time layer=3 col=speed val="5.0" where="label='unimproved road'"
       # define traveling costs as traveling time in minutes:
       # set forward costs
       v.db.update myroads_net_time layer=3 col=cost val="length / speed * 60"
       # set backward costs
       v.db.update myroads_net_time layer=3 col=bcost val="length / speed * 60"
       # ... the 'start' and 'end' nodes have category number 1 and 2
       # Shortest path: ID as first number, then cat1 and cat2
       echo "1 1 2" | v.net.path myroads_net_time alayer=3 nlayer=2 out=mypath
       # Fastest path: ID as first number, then cat1 and cat2
       echo  "1  1  2"  |  v.net.path  myroads_net_time  alayer=3 nlayer=2 afcol=cost abcol=bcost
       out=mypath_time
        To display the result, run for example:
       g.region vect=myroads_net
       d.mon x0
       d.vect myroads_net
       # show shortest path
       d.vect mypath col=red width=2
       # show fastest path
       d.vect mypath_time col=blue width=2
       # start and end point
       d.vect myroads_net icon=basic/triangle fcol=green size=12 layer=2
       d.font font=Vera
       d.vect startend disp=cat type=point lsize=14 layer=2

AUTHOR

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

       Last changed: $Date: 2013-05-23 13:01:55 -0700 (Thu, 23 May 2013) $

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