[GRASS-dev] [grass-code I][384] v.overlay with 'and' operator sometimes produces wrong results

[grass-codei at wald.intevation.org]

code I item #384, was opened at 2007-04-26 17:48
Status: Open
Priority: 3
Submitted By: Maciej Sieczka (msieczka)
Assigned to: Nobody (None)
Summary: v.overlay with 'and' operator sometimes produces wrong results 
Issue type: module bug
Issue status: None
GRASS version: CVS HEAD
GRASS component: vector
Operating system: all
Operating system version: 
GRASS CVS checkout date, if applies (YYMMDD): 070425


Initial Comment:
(this is a report moved from the old GRASS bug tracker [1] on a request of the submitter; the RT ticket [1] is now set to dead)



My name is Andras Fabian, and I use GRASS for a flight simulator project (see
more info under http://www.alpilotx.de/documentation.html)
Now I have observed, that in some rare situations, the following command 

"v.overlay -t ainput=pre_extract atype=area alayer=1 binput=biogeo_clip btype=area blayer=1 output=exportAlpine operator=and olayer=1,0,0 --overwrite"

doesn't produce the expected result. To be more exact, it seems as if it turns
one of the polygons inside out. To use the above command: pre_extract is a bunch of - mostly - smaller polygons already cut to a 1x1 degree tile.

biogeo_clip is another large (usually only one) polygon, which was clipped to
a 1x1 degree area (so essentially biogeo_clip often has 2-3 flat sides from 1x1
clipping area and the rest is some decently jagged line). 
So what I want with my command is: I would like to have all polygons from pre_extract which are inside biogeo_clip. This almost allays works - but not every time.

And my observation is, that this happens in the rare situation when some boundary lines of biogeo_clip exactly overlap with pre_extract.

If you need a test dataset (I suppose), which reproduces this behavior, please
let me know. I have put it aside (copied the folders out of /vector subfolder
of my working set) - I also saved the "bad result" exportAlpine. And this test
is nice, because it is only only some 200Kb in size.

[1]http://intevation.de/rt/webrt?serial_num=5485


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