v.net(1grass) GRASS GIS User's Manual v.net(1grass)
NAME
v.net - Performs network maintenance.
KEYWORDS
vector, network, network maintenance
SYNOPSIS
v.net
v.net --help
v.net [-cs] [input=name] [points=name] [output=name] opera-
tion=string [arc_layer=string] [arc_type=string[,string,...]]
[node_layer=string] [threshold=float] [file=name]
[turn_layer=string] [turn_cat_layer=string] [--overwrite] [--help]
[--verbose] [--quiet] [--ui]
Flags:
-c
Assign unique categories to new points
For operation ’nodes’
-s
Snap points to network
For operation ’connect’. By default, a new line from the point to
the network is created.
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
input=name
Name of input vector line map (arcs)
Required for operation ’nodes’, ’connect’, ’report’ and ’nreport’
points=name
Name of input vector point map (nodes)
Required for operation ’connect’ and ’arcs’
output=name
Name for output vector map
operation=string [required]
Operation to be performed
Options: nodes, connect, arcs, report, nreport, turntable
nodes: new point is placed on each node (line end) if doesn’t exist
connect: connect still unconnected points to vector network by in-
serting new line(s)
arcs: new line is created from start point to end point
report: print to standard output {line_category start_point_cate-
gory end_point_category}
nreport: print to standard output {point_category line_cate-
gory[,line_category...]}
turntable: create turntable on vector network
arc_layer=string
Arc layer
Vector features can have category values in different layers. This
number determines which layer to use. When used with direct OGR ac-
cess this is the layer name.
Default: 1
arc_type=string[,string,...]
Arc type
Input feature type
Options: line, boundary
Default: line,boundary
node_layer=string
Node layer
Vector features can have category values in different layers. This
number determines which layer to use. When used with direct OGR ac-
cess this is the layer name.
Default: 2
threshold=float
Threshold
Required for operation ’connect’. Connect points in given thresh-
old.
file=name
Name of input file
Required for operation ’arcs’ (’-’ for standard input)
turn_layer=string
Turntable layer
Layer where turntable will be attached. Format: layer number[/layer
name].Required for operation ’turntable’.
Default: 3
turn_cat_layer=string
Layer with unique categories used in turntable
Layer with unique categories for every line in arc_layer and point
on every node. The categories are used in turntable. Format: layer
number[/layer name]. Required for operation ’turntable’.
Default: 4
DESCRIPTION
v.net is used for network preparation and maintenance. Its main use is
to create a vector network from vector lines (arcs ) and points (nodes)
by creating nodes from intersections in a map of vector lines (node op-
erator), by connecting a vector lines map with a points map (connect
operator), and by creating new lines between pairs of vector points
(arcs operator).
A GIS network consists of topologically correct lines (arcs). That is,
the lines must be connected by shared vertices where real connections
exist. In GRASS GIS you also can add nodes to the network. These are
specially designated vertices used for analyzing network properties or
computing cost/distance measures. That is, not all vertices are treated
as nodes by default. Only v.net.path can use a network without nodes,
they are required for all the other network modules. In GRASS, net-
work arcs are stored in one data layer (normally layer 1) and nodes are
stored in a different data layer (normally layer 2).
v.net offers two ways to add nodes to a network of arcs and one method
to add arcs to a set of nodes:
1 Use the connect operation to create nodes from a vector points
file and add these nodes to an existing vector network of arcs
(i.e., lines/boundaries). This is useful when the goal is to an-
alyze a set of places (points) in relation to a network--for ex-
ample travel costs between places. Only points within the thresh
(threshold) distance to a line/boundary will be connected as
network nodes. There are two ways to connect nodes. By default,
v.net will create new lines connecting each point to the closest
line of the network. If you use the -s flag, however, the new
nodes will be added on the closest line of the network at the
point closest to the point you wish to add. When using the con-
nect operation, some lines will share the same category. In or-
der to assign unique costs to each line, a new layer needs to be
created with
v.category input=yourmap option=add cat=1 step=1 layer=3 out-
put=newmap
followed by
v.db.addtable map=newmap layer=3 table=tablename.
2 Create nodes and arcs from a vector line/boundary file using the
node operation. This is useful if you are mostly interested in
the network itself and thus you can use intersections of the
network as start and end points. Nodes will be created at all
intersections of two or more lines. For an arc that consists of
several segments connected by vertices (the typical case), only
the starting and ending vertices are treated as network nodes.
3 Create straight-line arcs between pairs of nodes with the arcs
option. This produces networks like those of airline flights be-
tween airports. It is also similar to the kind of network cre-
ated with social networking software, making it possible to cre-
ate georeferenced social networks.
While the arcs created with v.net will retain any attribute information
associated with the input vector line/boundary file in data layer 1,
nodes created and stored in data layer 2 will not have any associated
attribute information.
For nodes created using the connect and arcs operations (methods 1 and
3 above), the nodes can be reconnected to the attribute table of the
input vector points file using the attribute table manager ("manage
layers" tab) or by running v.db.connect.
For nodes created using the nodes operation (method 2 above), it is
possible to create an attribute table for the new nodes in layer 2 us-
ing the attribute table manager and connect it to layer 2 ("manage lay-
ers" tab) or to create a table with v.db.addtable, connect it to layer
2 with v.db.connect, and update the new table with cat values with
v.to.db.
The turntable operation creates a turntable with the costs for every
possible turn on every possible node (intersection, crossroad) in given
layer (arc_layer). U-turns are taken in account too. Turntable is
created in turn_layer and turn_cat_layer. Building the turntable al-
lows you to model e.g. traffic code, where some turns may be prohib-
ited. If features in analyzed network are changed, the turntable must
be created again (e.g. it includes v.net connect operation). Turntable
name consists of output vector map name + "_turntable_" + "t" + "_" +
turn_layer + "_" + "tuc" + "_" + turn_cat_layer + "_" + "a" + "_" +
arc_layer e. g. roads_turntable_t_3_tuc_4_a_1
These modules are able to work with the turntable: v.net.alloc,
v.net.iso, v.net.path, v.net.salesman For more information about turns
in the vector network analyses see wiki page.
Once a vector network has been created, it can be analyzed in a number
of powerful ways using the suite of v.net.* modules. The shortest
route between two nodes, following arcs, can be computed (v.net.path),
as can the shortest route that will pass through a set of nodes and re-
turn to the starting node (v.net.salesman). Least cost routes through
the network can be calculated on the basis of distance only or on the
basis of distance weighted by an attribute associated with each arc
(for example, travel speed along a network segment). A network can be
divided into concentric zones of equal travel cost around one or more
nodes (v.net.iso) or subdivided so that each node is surrounded by a
zone in which all arcs can be reached with the same travel costs as all
arcs surrounding each other node (v.net.alloc). In addition to the
modules listed above, the GRASS vector networking suite includes numer-
ous other modules for analysis of network costs and connectivity. These
include: v.net.allpairs, v.net.bridge, v.net.centrality, v.net.compo-
nents, v.net.distance, v.net.flow, v.net.spanningtree, v.net.steiner,
v.net.timetable, and v.net.visibility.
NOTES
For a vector map prepared for network analysis in GRASS, nodes are rep-
resented by the grass-internal geometry type node and arcs by the geom-
etry type line. If vector editing is required to modify the graph,
g.gui.vdigit or v.edit can be used. See also the Linear Referencing
System available in GRASS GIS.
EXAMPLES
The examples are North Carolina dataset based.
Create nodes globally for all line ends and intersections
v.net input=streets_wake output=streets_node operation=nodes
# verify result
v.category streets_node option=report
Merge in nodes from a separate map within given threshold
v.net input=streets_wake points=firestations out=streets_net \
operation=connect threshold=500
# verify result
v.category streets_net option=report
The nodes are stored in layer 2 unless node_layer=1 is used.
Generating network for vector point map
For generating network for given vector point map an input file in the
following format is required:
[category of edge] [category of start node] [category of end node]
Option 1: Save the file (e.g. "points.txt") and generate the map:
v.net points=geodetic_swwake_pts output=geodetic_swwake_pts_net \
operation=arcs file=points.txt
# verify result
v.category geodetic_swwake_pts_net option=report
Option 2: Read in from command line:
v.net points=geodetic_swwake_pts output=geodetic_swwake_pts_net \
operation=arcs file=- << EOF
1 28000 28005
2 27945 27958
3 27886 27897
EOF
# verify result
v.category geodetic_swwake_pts_net option=report
Generating network with turntable for vector point map
Following example generates a vector map with turntable:
v.net operation=turntable in=railroads out=railroads_ttb
SEE ALSO
g.gui.vdigit, v.edit, Vector Network Analysis Tool
v.net.alloc, v.net.allpairs, v.net.bridge, v.net.centrality,
v.net.components, v.net.connectivity, v.net.distance, v.net.flow,
v.net.iso, v.net.path, v.net.salesman v.net.spanningtree,
v.net.steiner, v.net.timetable, v.net.visibility
AUTHORS
Radim Blazek, ITC-irst, Trento, Italy
Martin Landa, FBK-irst (formerly ITC-irst), Trento, Italy and CTU in
Prague, Czech Republic (operation ’connect’ and ’arcs’)
Markus Metz: important fixes and improvements
TURNS SUPPORT
The turns support was implemnented as part of GRASS GIS turns cost
project at Czech Technical University in Prague, Czech Republic.
Eliska Kyzlikova, Stepan Turek, Lukas Bocan and Viera Bejdova partici-
pated at the project. Implementation: Stepan Turek Documentation:
Lukas Bocan Mentor: Martin Landa
SOURCE CODE
Available at: v.net source code (history)
Accessed: unknown
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GRASS 7.8.7 v.net(1grass)
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