[GRASS-user] v.overlay missing polygon

[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
> 
> 
> 
> 
> 
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