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NAME

KEYWORDS

SYNOPSIS

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

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

EXAMPLES

Create nodes globally for all line ends and intersections:

v.net input=streets_wake output=streets_node operation=nodes

v.net input=streets_wake points=firestations out=streets_net \
      operation=connect thresh=500

[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

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

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

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