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
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, Markus Metz
Last changed: $Date: 2013-05-23 22:01:55 +0200 (Thu, 23 May 2013) $
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