[GRASS-user] Removing small areas using v.clean

[Pierre Roudier]

Markus,

Thanks, that means this is no bug then.

However, in my specific case this behaviour is problematic (I do need
to merge small areas into existing/real areas), so maybe a suggestion
could be to control that behaviour through an option? Anyway, as I'm
not able to code it myself in C, so this is just a wish!

Thanks for the time you spent looking into my problem, this is much
appreciated as usual,

Pierre

2011/9/22 Markus Metz <markus.metz.giswork at googlemail.com>:
> On Thu, Sep 22, 2011 at 5:26 AM, Pierre Roudier
> <pierre.roudier at gmail.com> wrote:
>> BTW I confirm I got similar problems with the 6.4 series,
>>
> Thanks for the input vector, I could reproduce the effect. But I am
> not sure if this is really a problem. The method to remove small areas
> finds the neighbouring area with which the longest boundary is shared.
> This neighbouring area can also be the surrounding "nothing" in which
> case the area is not merged with a real area but erased. That is, my
> first comment was wrong, areas are not always merged with a
> neighbouring real area, only if this neighbouring real area is the one
> with the longest shared boundary. The cleaning tool assumes that small
> areas are spurious and removes them by merging or by erasing. In case
> of ties, when there are two neighbours with equally longest shared
> boundary, as in your case, whichever neighbour is checked first will
> be used. That neighbour can also be the empty space around.
>
> Markus M
>
>
>>
>> 2011/9/22 Pierre Roudier <pierre.roudier at gmail.com>:
>>> Thanks Markus,
>>>
>>> Here's an archive containing the input vector layer, along with
>>> v.clean output:
>>> https://df.bestgrid.org/quickshare/fb0bb3af778c8a03/vectors.tar.gz
>>>
>>> Here's the result of v.info -h on the output:
>>>
>>> COMMAND: r.to.vect input="c_means" output="foo" feature="area"
>>> GISDBASE: /data/GRASSDATA
>>> LOCATION: NZTM2000 MAPSET: BH USER: roudierp DATE: Thu Sep 15 17:29:53 2011
>>> ---------------------------------------------------------------------------------
>>> COMMAND: v.clean input="foo" layer="1"
>>> type="point,line,boundary,centroid,area,face,kernel" output="bar"
>>> tool="rmar
>>> ea" thresh=900
>>> GISDBASE: /data/GRASSDATA
>>> LOCATION: NZTM2000 MAPSET: BH USER: roudierp DATE: Thu Sep 15 17:31:45 2011
>>> [Raster MASK present]
>>>
>>> Cheers,
>>>
>>> Pierre
>>>
>>> 2011/9/21 Markus Metz <markus.metz.giswork at googlemail.com>:
>>>> On Wed, Sep 21, 2011 at 12:51 AM, Pierre Roudier
>>>> <pierre.roudier at gmail.com> wrote:
>>>>> Markus,
>>>>>
>>>>> Sorry for this late reply as I was away from keyboard the last few days,
>>>>>
>>>>> I encountered this problem while testing the latest grass_svn  (grass
>>>>> 7). I can give it a shot using a stable version if you want,
>>>>
>>>> You can try, but I would be surprised if the result is any different.
>>>> Can you make the vector available for testing, or can you reproduce
>>>> this with a vector of the sample datasets?
>>>>
>>>> Markus M
>>>>>
>>>>> 2011/9/17 Markus Metz <markus.metz.giswork at googlemail.com>:
>>>>>> On Fri, Sep 16, 2011 at 6:28 AM, Pierre Roudier
>>>>>> <pierre.roudier at gmail.com> wrote:
>>>>>>> Dear list,
>>>>>>>
>>>>>>> I got a problem as I'm trying to remove small areas of a vector layer
>>>>>>> using v.clean with the rmarea tool: some of these small areas that are
>>>>>>> on the boundary of the layer are getting erased rather than being
>>>>>>> merged with the closest polygon.
>>>>>>>
>>>>>>> Is that an expected behaviour?
>>>>>>
>>>>>> No, they should always be merged with a neighbouring area along the
>>>>>> shared boundary. Areas are supposed to be erased only if they are
>>>>>> isolated, i.e. do not share a boundary with another area. What GRASS
>>>>>> version are you using? There was a bug which was fixed 2 years ago.
>>>>>>
>>>>>> Markus M
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Scientist
>>>>> Landcare Research, New Zealand
>>>>>
>>>>
>>>
>>>
>>>
>>> --
>>> Scientist
>>> Landcare Research, New Zealand
>>>
>>
>>
>>
>> --
>> Scientist
>> Landcare Research, New Zealand
>>
>



-- 
Scientist
Landcare Research, New Zealand