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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]   operation=string   [arc_layer=string]
       [node_layer=string]   [threshold=float]   [file=name]   [--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
           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)
           arcs: new line is created from start point to end point
           report: print to standard output {line_category start_point_category end_point_category}
           nreport: print to standard output {point_category line_category[,line_category...]}

       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 access this is the layer name.
           Default: 1

       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 access this is the layer name.
           Default: 2

       threshold=float
           Threshold
           Required for operation ’connect’. Connect points in given threshold.

       file=name
           Name of input file
           Required for operation ’arcs’ (’-’ for standard input)

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 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
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. 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 connect operation, some lines will share the same category. In order to
assign unique costs to each line, a new layer needs to be created with
v.category map=yourmap operation=add cat=1 step=1 layer=3
followed by
v.db.addtable map=yourmap 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 between airports. It is also similar to the kind of network created with
social networking software, making it possible to create 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 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,  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 operation=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 operation=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 operation=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 operation=report

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: 2015-10-27 16:43:43 +0100 (Tue, 27 Oct 2015) $

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