[GRASS-dev] [GRASS GIS] #2648: v.concave.hull fails with small number of points

[GRASS GIS]

#2648: v.concave.hull fails with small number of points
--------------------------+--------------------------
  Reporter:  annakrat     |      Owner:  grass-dev@…
      Type:  defect       |     Status:  closed
  Priority:  normal       |  Milestone:  7.0.1
 Component:  Addons       |    Version:  unspecified
Resolution:  fixed        |   Keywords:  concave hull
       CPU:  Unspecified  |   Platform:  All
--------------------------+--------------------------
Changes (by mlennert):

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


Comment:

 Replying to [comment:6 annakrat]:
 > Replying to [comment:5 mlennert]:
 > > Replying to [comment:3 mlennert]:
 > > > Replying to [ticket:2648 annakrat]:
 > > > > Tested with 3, 4, 5 points, v.concave.hull fails. Depends on
 threshold and point configuration, too.
 > > > >
 > > > > {{{
 > > > > ERROR: Input vector map contains no boundaries.
 > > > > ...
 > > > >  ['v.centroids', '--q',
 'input=concave_hull_tmp_31390_delaunay_borders_select',
 'output=concave_hull_tmp_31390_delaunay_areas_select'] ended with error
 > > > > }}}
 > > >
 > > > AFAICT, the algorithm cannot calculate the hull because the
 threshold is too high. For now I've added a fatal error in the script with
 a hint about the threshold (r65495).
 > >
 > > I've changed it again in r65496: the algorithm now automatically
 reduces the threshold if it is too high and only fatals out if even a
 threshold of 0 wouldn't work.
 >
 > Thanks, this works for the case. But for some point configurations, the
 algorithm still doesn't produce what I would expect, for example these
 points where the concave and convex hull should be the same I think, but
 are not:
 >
 > {{{
 >  echo "638597|220531
 > 638838|220526
 > 638757|220237
 > 638606|220331
 > 638651|220576.08
 > 638735|220601.28
 > 638798|220578.18
 > 638856.8|220456.38
 > 638886.2|220292.58
 > 638644.7|220256.88
 > 638579.6|220437.48" | v.in.ascii in=- out=points --o
 >  v.concave.hull input=points out=hull --o
 > }}}
 >

 The PostGIS ST_ConcaveHull fonction authors agree with you as whatever the
 precentage I set, I always get the convex hull as a result.

 > I don't know what could be done about it.

 I don't either. I don't even know if there is one "correct" definition of
 a concave hull.


 > Maybe we could consider this ticket as solved. Thank you
 >

 Yes, I'm closing it. If you want to discuss the general results of
 v.concave.hull, thus probably warrants another ticket.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2648#comment:7>
GRASS GIS <http://grass.osgeo.org>