[GRASS-user] generalize polygons including holes

[Moritz Lennert]

On 05/02/15 14:49, Markus Metz wrote:
> On Thu, Feb 5, 2015 at 11:50 AM, Moritz Lennert
> <mlennert at club.worldonline.be> wrote:
>> On 05/02/15 10:06, Markus Metz wrote:
>>>
>>> On Thu, Feb 5, 2015 at 9:36 AM, Moritz Lennert
>>> <mlennert at club.worldonline.be> wrote:
>>>>
>>>> [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.
>>>
>>>
>>> The right hole has not been generalized properly, the resultant
>>> boundary intersects with itself. This has been fixed in GRASS 7
>>> recently. Do you get the same result with recent GRASS 7.1?
>>
>>
>> No. The example was with GRASS 7.0 from a week or two ago.
>
> OK, the test was missing in 7.0, I have now backported it to 7.0 in r64471
>
>> In GRASS 7.1 no
>> more intersection, but actually for that hole the generalization is less
>> with a higher threshold (see attached image).
>
> That hole is actually not generalized at all with thresh=500 because
> it would damage topology.

Maybe a little message would be helpful here to inform the user about this ?

>
>> I've attached the vector file
>> I used as a GRASS 7 vector pack for the NC location.
>>
>> The settings I used were:
>>
>> v.generalize test method=douglas thresh=100 out=test_gen_100
>> v.generalize test method=douglas thresh=500 out=test_gen_500
>>
>>>
>>> v.generalize does not distinguish between outer and inner rings, it
>>> simply goes through all boundaries and generalizes each one.
>>
>>
>> And this has always been the case or this is specific to GRASS7 ?
>
> This has always been the case.

Then why doesn't there seem to be any generalization happening for the 
hole boundaries of the test dataset with grass64release ?

Moritz