## NAME

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: $Date$

## SOURCE CODE

Available at: v.net.visibility source code (history)

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© 2003-2019 GRASS Development Team, GRASS GIS 7.8.0 Reference Manual