[GRASS-user] v.overlay missing polygon

[Falko Engel]

Am 12.03.2012 17:28, schrieb Vincent Bain:
> Falko, just an idea :
> for theory 2, perhaps can you try to clean either a or b map, by
> snapping boundaries vertices over each other :
> v.edit map=a bgmap=b type=boundary tool=snap snap=vertex thres=...
>
>
> Vincent.
>
>
> Le lundi 12 mars 2012 à 17:11 +0100, Falko Engel a écrit :
>> Dear List,
>>
>> I have encountered some problems when using v.overlay on polygon maps
>> (and also v.patch): There are some polygons missing in the output map
>> that should not be missing. And there are polygons there that should not
>> be there.
>>
>>
>> I have two theories for this problem:
>>
>> 1. The input data (either ainput or binput or both) has geometry errors
>>
>> 2. Data set ainput and binput "almost" share some boundaries.
>> This can happen, when someone creates an new vector map and uses an
>> existing one as a basis. He would then digitize manually along a
>> boundary of the base map and produce an inexactly overlaying new boundary.
>> My theory in this case is that when using v.overlay on these two data
>> sets the inexactly created boundaries result in many very small
>> intersections which again result in wrong geometries.
>> I'm not sure about this, it's just a theory.
>>
>>
>> In my case I have used the following command to overlay my vector maps:
>>
>> v.overlay ainput=veg atype=area binput=plot btype=area
>> output=res_overlay operator=and
>>
>> You can find the data I have used at:
>>
>> http://dl.dropbox.com/u/1542545/data_overlay.zip
>>
>> My system components are:
>> GIS: GRASS GIS 6.4.1
>> OS: openSUSE 11.3 (x86_64)
>>
>>
>> I checked the geometries using ftools in QGIS. There seem to be no
>> geometry errors in these data sets. So I am focused on my second theory.
>> I use GRASS GIS in a quite large project where I can't choose the data
>> sets I am working with. I came across this problem more than once so far
>> and I hope that I am missing something or that there is a solution to this.
>>
>> I would be thankful if someone could spare the time and have a look at
>> my data.
>>
>> Falko
>>
>>
>>
>>
>>
>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>
>
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user

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