[QGIS-trac] [Quantum GIS] #2497: ftools: difference tool (asymmetrical difference) does not work with moltipolygon geometries

[Quantum GIS]

#2497: ftools: difference tool (asymmetrical difference) does not work with
moltipolygon geometries
------------------------------------------------+---------------------------
   Reporter:  lutra                             |              Owner:  cfarmer      
       Type:  bug                               |             Status:  new          
   Priority:  major: does not work as expected  |          Milestone:  Version 1.5.0
  Component:  Python plugins and bindings       |            Version:  HEAD         
   Keywords:                                    |   Platform_version:               
   Platform:  All                               |           Must_fix:  Yes          
Status_info:  0                                 |  
------------------------------------------------+---------------------------
 When trying to make a "difference" operation between polygon layers and
 both are multipolygon then the following warning is issued

 {{{
 Warnings:
 GEOS geoprocessing error: One or more input features have invalid
 geometry.
 Some output geometries may be missing or invalid.
 }}}

 and the resulting geometry is wrong (empty).

 The same happens if the diffence layer is multipart and the input layer
 singlepart, while otherwise -or with both input/difference layers as
 singlepart- acceptable results are given and no GEOS error messages are
 thrown.

 In general I would suggest to add a control and don't allow to use
 multipart geometries in this operation in order to avoid problems.

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/2497>
Quantum GIS <http://qgis.org>
Quantum GIS is an Open Source GIS viewer/editor supporting OGR, PostGIS, and GRASS formats