[GRASS-user] v.overlay missing polygon

[Falko Engel]

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