[GRASS-user] v.overlay missing polygon

[Falko Engel]

Am 13.03.2012 07:53, schrieb Vincent Bain:
> Falko,
> if you try to make things match, looks like v.overlay returns a correct
> result. Try the following command -- threshold 5 m is arbitrary, of
> course :
>
> v.edit map=plot type=boundary tool=move move=0,0 thresh=-1,5,0 bgmap=veg
> snap=vertex bbox=3607160,5733860,3609000,57350
>
> then
> v.overlay --o ainput=veg atype=area binput=plot btype=area
> output=res_overlay operator=and
>
> (right now, I can't tell you what was wrong for the long western
> polygon)
>
> Vincent.
>
>
>
> Le mardi 13 mars 2012 à 11:41 +1100, Richard Chirgwin a écrit :
>> On 13/03/12 6:36 AM, grass-user-request at lists.osgeo.org wrote:
>>> Dear Vincent,
>>>
>>> thanks for your idea. I wasn't aware of the possibility to snap a vector
>>> map to another one. Good thing to know!
>>> However, it didn't solve my problem.
>>> Actually, I checked the data with respect to my theory 2 again, and I
>>> must say: Visually it seems, that in this case the two data sets I would
>>> like to overlay match perfectly concerning the boundaries. The theory
>>> might be wrong.
>>>
>>>
>>> I did some further testing:
>>>
>>> Even if I isolate the polygon that disappears in the overlay process
>>>
>>> v.extract input=veg output=veg_137 list=137
>>>
>>> and use it for the overlay
>>>
>>> v.overlay ainput=plot atype=area binput=veg_137 btype=area operator=and
>>> output=res_137_and
>>>
>>> the result is erroneous. Nothing new, but still very strange.
>>>
>>>
>>> Tricky. I am puzzled and need to get home;-)
>>>
>>> Falko
>> Falko,
>>
>> I agree - the same error reproduces for me, using your vectors.
>>
>> Neither veg nor plot show any apparent errors in v.digit.
>>
>> Anyone offer suggestions to try and debug this?
>>
>> Richard
>>
>>
>>
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Dear Vincent,

I was able to reproduce your code. This is definitely an improvement! 
For the test data this solution works.
I applied the snapping code to a larger data set. There was only (or 
still) one case left where a polygon dissapeared. I guess the reason 
was, that there was a problematic overlap but no vertices within the 
snapping threshold. In this case it would be desirable that snapping 
could also be done with boundaries.

I think a snapping option in the manner of your suggestion should be an 
option in the v.overlay and v.patch tools. Disappearing polygons are 
really problematic for my work and this could be a solution.

Actually a friend told me that in PostGIS there is an option to snap all 
vector data to a grid. This would be possible in Grass if you would 
produce a vector grid (maybe from a raster file with an appropriate 
resolution) and use it as a bgmap. Did you ever try this?

Falko