[GRASS-dev] [GRASS GIS] #2492: v.generalize: incorrect handling of wrap at 180 meridian

GRASS GIS trac at osgeo.org
Tue Nov 18 02:42:18 PST 2014

#2492: v.generalize: incorrect handling of wrap at 180 meridian
 Reporter:  jkaplan       |       Owner:  grass-dev@…              
     Type:  defect        |      Status:  new                      
 Priority:  normal        |   Milestone:                           
Component:  Vector        |     Version:  svn-trunk                
 Keywords:  v.generalize  |    Platform:  All                      
      Cpu:  OSX/Intel     |  
 I am running GRASS 7.1.svn on Mac OS X.

 GRASS 7.1.svn (global_lonlat):~ > g.version -g

 I am having a problem with the behavior of v.generalize when attempting to
 generalize a vector feature - lines or polygons it does not seem to matter
 - that crosses the 180 meridian, using a location that is EPSG:4326 with
 global extents.

 The problem is probably best illustrated with a picture, which I have
 attached to this ticket, but can also be viewed here:


 In the picture two line vector features are displayed. The black-lined
 feature is the original vector line feature (test1). The red-lined feature
 is produced from test1 using the following command:

 v.generalize input=test1 output=test2 method=sliding_averaging
 look_ahead=5 slide=1 threshold=1

 It appears that v.generalize, when smoothing between vertices that are
 very close to the 180 meridian, results in the incorrect identification of
 the “next" point, hence the long horizontal lines connecting points “the
 wrong way around” the earth. The behavior is the same when using any of
 the generalization or smoothing tools.

 I have tested this with a simpler vector containing points farther away
 from the 180 meridian and it doesn’t appear to show the same behavior.

 I would very much appreciate any help anyone could give me to solve this

 Thank you in advance!

 Best regards,

 Jed Kaplan


 If you would like to download the original lines file and try my procedure
 yourself, I have also attached a test vector file to this ticket. It can
 also be downloaded here:


 Assuming you have OGR support for GMT in your version of GRASS, you can
 import the vector using this command:

 v.in.ogr dsn=GRASS_wrap_problem_samplevector.gmt output=test1

Ticket URL: <http://trac.osgeo.org/grass/ticket/2492>
GRASS GIS <http://grass.osgeo.org>

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