v.net.visibility.1grass man page
v.net.visibility — Performs visibility graph construction.
Keywords
vector, network, shortest path, visibility
Synopsis
v.net.visibility
v.net.visibility --help
v.net.visibility input=name output=name [coordinates=east,north] [visibility=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags
- --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 [required]
Name of input vector map
Or data source for direct OGR access- output=name [required]
Name for output vector map
- coordinates=east,north
Coordinates
- visibility=name
Name of input vector map containing visible points
Add points after computing the visibility graph
Description
v.net.visibility computes the visibility graph of a vector map containing lines, areas (boundaries) and points. The visibility graph is the graph where the nodes are the end point of the lines, boundaries or simply points. There is an edge between two nodes if they are ’visible’ to each other. Two nodes are visibible if there are no segments in between of them, i.e. the edge does not intersect any line or boundary in the vector map. This is useful to compute the shortest path in a vector map from any two points. To do this, first you need to compute the visibility graph and from that to compute the shortest path using v.net.path or d.path.
IMPORTANT: the algorithm doesn’t work well with intersecting lines (that includes overlapping)
Notes
If you compute a shortest path after computing the visibility graph you will notice that this path might go through a vertex of a line. If this is not what you wanted you might need to process the map in v.buffer, initially with a small value. Example:
v.buffer input=map output=bufferedmap buffer=1 type=point,line,area,boundary
The first argument is the input map. It supports lines, boundaries (so, areas) and points. For the algorithm was written to work with lines and boundaries not intersecting each other (that includes overlapping).
The resulting map containing the visibility graph is given in the output map.
If you need to add additional points to compute a shortest path between them afterwards you can use the coordinates parameter, e.g.:
coordinates=25556200,6686400,25556400,6686600
where 25556200,6686400 are the coordinate of the first point and 25556400,6686600 are the coordinates of the second point. Of course you can give as many points as you need. They will be added to the visibility graph and edges from them will be computed. You can always add those points after computing the visibility graph. Simply use the visibility parameter. The input will be the original vector map, the vis will be the computed visibility graph and the output the new visibility graph which will be the vis + the new points given with coordinate (edges will be computed as well).
v.net.visibility input=map visibility=vis_map output=new_vis_map \
coordinates=25556200,6686400,25556400,6686600
Examples
Example 1
A simple example (North Carolina sample data) showing how to use the module:
v.extract input=zipcodes_wake output=areas_7_11_25 cats=7,11,25
g.region vector=zipcodes_wake
d.mon wx0
d.vect areas_7_11_25
v.net.visibility input=areas_7_11_25 output=graph
d.vect graph
d.vect areas_7_11_25 color=red type=boundary
Example 2
An example on how to use v.buffer along with the module:
v.buffer input=lines output=buffered_lines buffer=1
v.net.visibility input=buffered_lines output=graph
d.vect graph
d.vect lines col=red
Example 3
An example on how to use the coordinate parameter. This will compute the visibility graph of the vector map lines with the point 2555678,6686343:
v.net.visibility input=lines output=graph coordinates=2555678,6686343
d.vect graph
d.vect lines col=red
Example 4
An example (North Carolina sample data) on how to use the coordinate parameter with the vis parameter. Here the vector map graph is computed then a new visibility graph is computed from it with the point 669547.97,208348.20 extra:
v.extract input=zipcodes_wake output=areas_7_11_25 cats=7,11,25
g.region vector=zipcodes_wake
d.mon wx0
d.vect areas_7_11_25
v.net.visibility input=areas_7_11_25 output=graph
v.net.visibility input=areas_7_11_25 visibility=graph output=new_graph \
coordinates=669547.97,208348.20
d.erase
d.vect areas_7_11_25
echo "symbol basic/star 20 669547.97 208348.20 black red" | d.graph -m
d.vect new_graph
d.vect areas_7_11_25 color=red type=boundary
Example 5
An example for connections of points (Spearfish):
v.net.visibility input=archsites output=graph
g.region vector=archsites
d.mon wx0
d.vect graph
d.vect archsites col=red
Example 6
Here is an example with artificial data.
Load data using here document syntax (Bash and unix-like commands lines only):
v.in.ascii input=- output=simple format=standard <<EOF
VERTI:
B 6
82.19908257 75.21788991
81.67889908 71.40321101
83.58623853 71.72522936
84.3293578 75.21788991
82.24862385 76.06009174
82.19908257 75.21788991
C 1 1
82.88897401 73.66318782
1 1
C 1 1
90.72645705 75.61248675
1 2
C 1 1
89.37944702 69.51012912
1 3
C 1 1
81.60108979 67.78669725
1 4
B 5
89.92752294 73.95458716
92.37981651 75.11880734
91.56238532 77.29862385
88.96146789 75.88669725
89.92752294 73.95458716
B 10
88.54036697 70.70963303
87.92192518 70.04087417
87.89633028 69.00045872
88.66460807 68.13372867
90.15108904 68.23290821
90.9426344 68.97588202
90.86880734 70.11513761
90.00144697 70.78336312
89.06055046 70.95733945
88.54036697 70.70963303
B 9
78.73119266 71.35366972
80.76238532 68.90137615
84.20550459 66.91972477
89.87798165 65.35917431
83.23494031 66.27685175
80.34278748 68.00837238
78.38484005 71.40292009
78.40917431 72.27018349
78.73119266 71.35366972
EOF
Compute the graph:
v.net.visibility input=simple output=graph
Known Issues
In some cases when 3 points or nodes are collinear, some wrong edges are added. This happens only really rarly and shouldn’t be a big problem. When two points have the exact same x coordinate and are visible, some wrong edges are added.
See Also
d.path, v.net, v.net.alloc, v.net.iso, v.net.salesman, v.net.steiner, v.to.db
Author
Maximilian Maldacker
Mentor: Wolf Bergenheim
Last changed: $Date: 2016-12-11 19:59:46 +0100 (Sun, 11 Dec 2016) $
Source Code
Available at: v.net.visibility source code (history)
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