[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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Comment(by mmetz):

 Replying to [comment:2 mlennert]:
 > 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
 }}}
 > > >
 > > >
 >
 > 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 ? ;-)

 Perfectly. Please try the new -s flag in r53665 for v.net to "Snap points
 to network". I think this makes sense for network preparation in general,
 particularly for features like schools, hospitals etc. that are
 represented by a point.

 With the above test command, -s flag added, there are now only two points
 with a betweenness of 0.

 Markus M

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1780#comment:3>
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