NAME¶
v.net - Performs network maintenance.
KEYWORDS¶
vector, networking, maintenance
SYNOPSIS¶
v.net
v.net help
v.net [-
c]
input=
name [
points=
name]
[
output=
name] [
operation=
string]
[
alayer=
integer] [
nlayer=
integer]
[
thresh=
float] [--
overwrite] [--
verbose]
[--
quiet]
Flags:¶
- -c
-
Assign unique categories to new points
For operation 'nodes'
- --overwrite
-
Allow output files to overwrite existing files
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
Parameters:¶
- input=name
-
Name of input vector map
- points=name
-
Name of input point vector map
Required for operation 'connect'
- output=name
-
Name for output vector map
- operation=string
-
Operation to be performed
Options: nodes,connect,report,nreport
Default: nodes
nodes: new point is placed on each node (line end) if doesn't exist
connect: connect still unconnected points to vector network by
inserting new line(s)
report: print to standard output {line_category start_point_category
end_point_category}
nreport: print to standard output {point_category
line_category[,line_category...]}
- alayer=integer
-
Arc layer
A single vector map can be connected to multiple database tables. This
number determines which table to use.
Default: 1
- nlayer=integer
-
Node layer
A single vector map can be connected to multiple database tables. This
number determines which table to use.
Default: 2
- thresh=float
-
Threshold
Required for operation 'connect'. Connect points in given threshold.
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 operator), 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, network 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:
- 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 analyze a set of
places (points) in relation to a network--for example travel costs between
places. Only points within the thresh (threshold) distance to a
line/boundary will be connected as network nodes.
- 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.
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 operation (method 1 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 using the
attribute table manager and connect it to layer 2 ("manage layers"
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.
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 return 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 numerous other modules for analysis of network costs
and connectivity. These include:
v.net.allpairs,
v.net.bridge,
v.net.centrality,
v.net.components,
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 represented
by the grass-internal geometry type
node and arcs by the geometry type
line. If vector editing is required to modify the graph,
wxGUI
vector digitizer 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
Merge in nodes from a separate map within given threshold:
v.net input=streets_wake points=firestations out=streets_net \
operation=connect thresh=500
For generating network for given vector point map is required input file in
format:
[category of edge] [category of start node] [category of end node]
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
SEE ALSO¶
wxGUI vector digitizer, v.edit
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
Last changed: $Date: 2013-05-23 22:01:55 +0200 (Thu, 23 May 2013) $
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