[GRASS-dev] [GRASS GIS] #1780: v.net.centrality: betweenness centrality seems wrong

[GRASS GIS]

#1780: v.net.centrality: betweenness centrality seems wrong
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  Reporter:  mlennert     |       Owner:  grass-dev@…                 
      Type:  defect       |      Status:  closed                      
  Priority:  normal       |   Milestone:  7.0.0                       
 Component:  Vector       |     Version:  svn-trunk                   
Resolution:  invalid      |    Keywords:  v.net.centrality betweenness
  Platform:  Unspecified  |         Cpu:  Unspecified                 
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Changes (by mlennert):

  * status:  new => closed
  * resolution:  => invalid


Comment:

 Replying to [comment:1 mmetz]:
 > Replying to [ticket:1780 mlennert]:
 >
 > >
 > {{{
 > v.net streets_wake points=schools_wake thresh=200 out=network op=connect
 >
 > v.net.centrality input=network alayer=1 nlayer=2 output=centrality
 cats=1-108 degree=degree closeness=closeness betweenness=betw
 eigenvector=eig
 > }}}
 > >
 > >
 > > Only three nodes (36, 136, 138) have betweenness centrality above 0
 which seems wrong.
 >
 > I get six nodes (28, 94, 97, 136, 138, 154) with betweenness centrality
 > 0.

 I can confirm these six nodes. Don't know what happened in the previous
 try.

 > For all other nodes, a betweenness centrality of 0 seems correct because
 they are located at end points of the network, i.e. not in between any two
 other nodes.
 >

 Ok, I see my error: since the schools are connected by short lines to the
 network, no shortest path between two other schools ever passes through
 those short lines and thus betweenness is 0. The only exception are those
 schools which are right next to another school and where the latter is
 connected to the network via the former. Then all shortest paths from the
 latter go through the former...

 This raises the question of whether it would be envisageable / desirable
 to add an option to v.net allowing to create nodes _on_ the existing
 network when connecting new points, instead of connecting these points by
 short lines. IOW, instead of currently adding a short line from any point
 in the given threshold to the network, "just" create a new node on a point
 of the network that is closest to the point you wish to add. Am I being
 clear ? ;-)

 Closing this bug as invalid.

 Moritz

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1780#comment:2>
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