[GRASS-user] generalize polygons including holes

[Moritz Lennert]

[Please keep conversations on the list]

On 04/02/15 16:12, Robert Nuske wrote:
> Hi Moritz
>
>>> is there a way to generalize polygons including their holes?
>>> I am not too worried about topology, there are only polygons of one class
>>> in the dataset.
>>>
>>>
>>> I tried the following
>>> v.generalize in=m3 out=m5 type=area method=douglas threshold=5000
>>>
>>> which simplified the "outer boundaries" okay but did not touch the "inner
>>> boundaries"/ holes of the polygon.
>>>
>>>
>>> Outer boundaries are on the left side in the picture and holes on the
>>> right
>>> http://i.imgur.com/hHKr15m.png
>>
>> It looks like the red boundaries are the generalized version of the
>> original blue ones. Is that correct ? Are you sure that the original
>> boundaries of the holes are identical to the new ones ? Could you
>> display the result with polygon fill color set to transparent ?
>
> That's right.
> Original vector map is blue and simplified vector is red.
> Both layers are 50% opaque in the following image
> http://i.imgur.com/Rsx7Kh1.png

I imagine you are using GRASS 6 ? I can confirm the issue there with 
GRASS 6.4.4, but it has been solved in GRASS 7. See the attached image 
where you have two holes: both have been generalized with method=douglas 
(black = original, red=lower threshold, blue=higher threshold). You can 
clearly see that generalization is also happening for the hole boundaries.

As GRASS 7 will come out really soon, and the current release candidates 
are quite stable, I recommend you switch to GRASS 7.

Moritz
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