3
<em>v.net</em> is used for network preparation and maintenance. Its
4
main use is to create a vector network from vector lines (<em>arcs
5
</em>) and points (<em>nodes</em>) by creating nodes from
6
intersections in a map of vector lines (<em>node</em> operator), by
7
connecting a vector lines map with a points map (<em>connect</em>
8
operator), and by creating new lines between pairs of vector points
9
(<em>arcs</em> operator).
12
A GIS network consists of topologically correct lines (arcs). That is,
13
the lines must be connected by shared vertices where real connections exist.
14
In GRASS GIS you also can add nodes to the network. These are specially
15
designated vertices used for analyzing network properties or computing
16
cost/distance measures. That is, not all vertices are treated as nodes by
17
default. Only <em><a href="v.net.path.html">v.net.path</a></em> can use a
18
network without nodes, they are required for all the other network modules.
19
In GRASS, network arcs are stored in one data layer (normally layer 1) and
20
nodes are stored in a different data layer (normally layer 2).
23
<em>v.net</em> offers two ways to add nodes to a network of arcs:
26
<li>Use the <em>connect</em> operation to create nodes from a vector points
27
file and add these nodes to an existing vector network of arcs (i.e.,
28
lines/boundaries). This is useful when the goal is to analyze a set of places
29
(points) in relation to a network--for example travel costs between places.
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Only points within the <em>thresh</em> (threshold) distance to a line/boundary
31
will be connected as network nodes.</li>
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<li>Create nodes and arcs from a vector line/boundary file using the <em>node</em>
34
operation. This is useful if you are mostly interested in the
35
network itself and thus you can use intersections of the network as start and
36
end points. Nodes will be created at all intersections of two or more lines.
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For an <em>arc</em> that consists of several segments connected by
38
vertices (the typical case), only the starting and ending vertices are
39
treated as network nodes.</li>
43
While the arcs created with v.net will retain any attribute information
44
associated with the input vector line/boundary file in data layer 1, nodes
45
created and stored in data layer 2 will not have any associated attribute
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For nodes created using the <em>connect</em> operation (method 1
50
above), the nodes can be reconnected to the attribute table of the
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input vector points file using the attribute table manager ("manage layers"
52
tab) or by running <em><a href="v.db.connect.html">v.db.connect</a></em>.
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For nodes created using the <em>nodes</em> operation
56
(method 2 above), it is possible to create an attribute table for the
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new nodes in layer 2 using the attribute table manager and connect it to
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layer 2 ("manage layers" tab) or to create a table with
59
<em><a href="v.db.addtable.html">v.db.addtable</a></em>,
60
connect it to layer 2 with <em><a href="v.db.connect.html">v.db.connect</a></em>,
61
and update the new table with cat values with <em><a href="v.to.db.html">v.to.db</a></em>.
64
Once a vector network has been created, it can be analyzed in a number
65
of powerful ways using the suite of <em>v.net</em>.* modules. The shortest route
66
between two nodes, following arcs, can be computed
67
(<em><a href="v.net.path.html">v.net.path</a></em>), as can the
68
shortest route that will pass through a set of nodes and return to the
69
starting node (<em><a href="v.net.salesman.html">v.net.salesman</a></em>).
70
Least cost routes through the network can be calculated on the basis of
71
distance only or on the basis of distance weighted by an attribute
72
associated with each arc (for example, travel speed along a network segment).
73
A network can be divided into concentric zones of equal travel cost around
74
one or more nodes (<em><a href="v.net.iso.html">v.net.iso</a></em>) or
75
subdivided so that each node is surrounded by a zone in which all arcs
76
can be reached with the same travel costs as all arcs surrounding each
77
other node (<em><a href="v.net.alloc.html">v.net.alloc</a></em>).
79
In addition to the modules listed above, the GRASS vector networking suite
80
includes numerous other modules for analysis of network costs and
81
connectivity. These include:
82
<em><a href="v.net.allpairs.html">v.net.allpairs</a></em>,
83
<em><a href="v.net.bridge.html">v.net.bridge</a></em>,
84
<em><a href="v.net.centrality.html">v.net.centrality</a></em>,
85
<em><a href="v.net.components.html">v.net.components</a></em>,
86
<em><a href="v.net.distance.html">v.net.distance</a></em>,
87
<em><a href="v.net.flow.html">v.net.flow</a></em>,
88
<em><a href="v.net.spanningtree.html">v.net.spanningtree</a></em>,
89
<em><a href="v.net.steiner.html">v.net.steiner</a></em>,
90
<em><a href="v.net.timetable.html">v.net.timetable</a></em>, and
91
<em><a href="v.net.visibility.html">v.net.visibility</a></em>.
95
For a vector map prepared for network analysis in GRASS, nodes are
96
represented by the grass-internal geometry type <em>node</em> and arcs
97
by the geometry type <em>line</em>.
99
If vector editing is required to modify the
100
graph, <em><a href="wxGUI.Vector_Digitizer.html">wxGUI vector
101
digitizer</a></em> or <em><a href="v.edit.html">v.edit</a></em> can be
103
See also the <a href="lrs.html">Linear Referencing System</a> available in
108
The examples are <a href="http://www.grassbook.org/data_menu3rd.php">North Carolina dataset</a> based.
110
<p>Create nodes globally for all line ends and intersections:
112
<div class="code"><pre>
113
v.net input=streets_wake output=streets_node operation=nodes
116
Merge in nodes from a separate map within given threshold:
118
<div class="code"><pre>
119
v.net input=streets_wake points=firestations out=streets_net \
120
operation=connect thresh=500
123
For generating network for given vector point map is required input
126
<div class="code"><pre>
127
[category of edge] [category of start node] [category of end node]
130
<div class="code"><pre>
131
v.net points=geodetic_swwake_pts output=geodetic_swwake_pts_net \
132
operation=arcs file=- << EOF
142
<a href="wxGUI.Vector_Digitizer.html">wxGUI vector digitizer</a>,
143
<a href="v.edit.html">v.edit</a>
148
<a href="v.net.alloc.html">v.net.alloc</a>,
149
<a href="v.net.allpairs.html">v.net.allpairs</a>,
150
<a href="v.net.bridge.html">v.net.bridge</a>,
151
<a href="v.net.centrality.html">v.net.centrality</a>,
152
<a href="v.net.components.html">v.net.components</a>,
153
<a href="v.net.connectivity.html">v.net.connectivity</a>,
154
<a href="v.net.distance.html">v.net.distance</a>,
155
<a href="v.net.flow.html">v.net.flow</a>,
156
<a href="v.net.iso.html">v.net.iso</a>,
157
<a href="v.net.path.html">v.net.path</a>,
158
<a href="v.net.salesman.html">v.net.salesman</a>
159
<a href="v.net.spanningtree.html">v.net.spanningtree</a>,
160
<a href="v.net.steiner.html">v.net.steiner</a>,
161
<a href="v.net.timetable.html">v.net.timetable</a>,
162
<a href="v.net.visibility.html">v.net.visibility</a>
167
Radim Blazek, ITC-irst, Trento, Italy<br>
169
Martin Landa, FBK-irst (formerly ITC-irst), Trento, Italy and CTU in
170
Prague, Czech Republic (operation 'connect' and 'arcs')<br>
172
Markus Metz: important fixes and improvements
174
<i>Last changed: $Date: 2013-05-23 22:01:55 +0200 (Thu, 23 May 2013) $</i>