[QGIS-trac] Re: [Quantum GIS] #3187: transform ring into new polygon

[Quantum GIS]

#3187: transform ring into new polygon
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        Reporter:  mlennert          |         Owner:  jef          
            Type:  enhancement       |        Status:  reopened     
        Priority:  minor: annoyance  |     Milestone:  Version 1.7.0
       Component:  fTools            |       Version:               
      Resolution:                    |      Keywords:               
Platform_version:                    |      Platform:  All          
        Must_fix:  No                |   Status_info:  0            
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Changes (by mlennert):

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


Comment:

 Replying to [comment:4 pcav]:
 > In trunk a "Lines to polygons" command has been added. Please check if
 this is what you need and reopne the ticket if not.

 Unless I don't understand your proposed solution, this is not a response
 to the original feature request. What I'm aiming at is something that will
 allow to easily digitize islands, i.e. smaller polygons comletely enclosed
 by larger polygons. QGIS already allows to create rings, i.e. holes in
 larger polygons. Thus the title of the feature request, as it might be a
 possible solution to allow the transformation of such rings into polygons.
 Other options would be possible, such as the possibility to declare a
 polygon as an island, thus "cutting" the larger polygon by the smaller
 polygon.

 Currently, if you digitize both polygons, they just overlap. So, in order
 to get an island, you have to digitize the larger polygon, cut a ring into
 this polygon, and then correctly use snapping in order to digitize the
 smaller polygon exactly on the contours of the ring. Not impossible, but
 tedious and error-prone.

 As mentioned in the comments, GRASS's data structure allows to do this
 almost automatically, as it does not allow overlapping features and so
 digitizing the smaller polygon over the larger polygon automatically cuts
 the hole into the larger polygon.

 As I said, several options are available to solve this problem:[[BR]]

 1) Allow declaring a ring as a new polygon, i.e. being able to say: this
 hole I just cut into my forest actually is a lake.[[BR]]

 2) Allow to cut a ring with a new polygon, i.e. being to able to say: this
 polygon I just digitized is my lake and it should cut a hole into my
 forest[[BR]]

 3) Allow to copy specific boundary lines, i.e. being able to say: take the
 boundaries of the hole I just cut into my forest and copy them as
 boundaries of a new polygon (my lake) I wish to create.[[BR]]


 Moritz

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Ticket URL: <http://trac.osgeo.org/qgis/ticket/3187#comment:5>
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