grass-dev@grass.itc.it wrote:
I have a set of vectory
polygons (call it "polygons_select") and one larger
vector polygon which almost overlaps the previous ones
(call it "eco_clipped_central_appalachian"). Now, I would
like to cut A by B, so I obviously do a v.overlay with
"and". Or as it looks in reality: " v.overlay
ainput=polygons_select atype=area alayer=1
binput=eco_clipped_central_appalachian btype=area
blayer=1 output=polygons_current operator=and
olayer=1,0,0 --overwrite " Now the resulting
"polygons_current" is missing some polygons, which should
obviously NOT be missing! See attached screenshots: -
polygons_select.jpg is where I start -
polygons_select_WITH_eco_clipped_central_appalachian.jpg
shows the second polygon (in transparent yellow) which I
would like to overlay with the first ones. -
polygons_current.jpg shows with the red arrow, where one
polygon is missing.
I've done some testing. I suspect the origin of the issue
might be a bug/feature in v.overlay that prevents it from
working reliably in lat/long locations. Please anybody
verify. Details follow.
I can reproduce the problem reported by Andras [1] with his
data [2], which are a lat/long location. Namely, the output of:
$ v.overlay ainput=polygons_select atype=area
binput=eco_clipped_central_appalachian btype=area
output=polygons_current operator=and
on my machine is a vector that has the same missing polygon
as on Andras' screendump [3].
Fiddling around I reprojected Andras' data into a metric
projection (the spearfish60 location), and the v.overlay
command as above worked perfect then. Not only the polygon
missing on [3] was there, but also the artifact described by
Andras in his another report [4] were not present.
In this case, can anybody tell if v.overlay is not supposed
to be used in lat/lon locations, or is it a bug that can be
fixed?
Maciek
[1]http://wald.intevation.org/tracker/index.php?func=detail&aid=452&group_id=21&atid=204
[2]http://wald.intevation.org/tracker/download.php/21/204/452/227/Testdata_location.tar.bz2
[3]http://wald.intevation.org/tracker/download.php/21/204/452/221/polygons_current.jpg
[4]http://wald.intevation.org/tracker/index.php?func=detail&aid=384&group_id=21&atid=204