I'm bringing this back to the list so others can comment/help. On 03/25/2013 09:22 AM, Hugues François wrote: > The shapefile contains only the simple polygons i would like to merge to make > a single (valid) multipolygon (simple ones come from an invalid multi dumped > into valid single parts). > > I hope this is clearer but i'm not really sure. With simple words simple > geometries are valid but their union is not despite its topology seems to be > ok. OK. So if I take your shape file and run Vector/Geometry Tools/Singlepart to multipart. Then run Vector/Geometry Tools/Check Validity on the results I too get the errors you mention. This sounds like a bug but I'm not a GIS expert so what do others think?
Note grass doesn't report any topo errors on either the singlepart or multipart shape files. > > Hugues > > "David J. Bakeman" <[email protected]> a écrit : > > On 03/25/2013 08:44 AM, Hugues François wrote: >> Hello, >> >> Thanks for your answer. I work with 1.8.0 lisboa on Windows. >> >> Did you use the same tool as i did to check geometry validity >> (vector/geometry tool/check validity) with a *merged* geometry from the 2 >> ones i sent ? > I did use the same tool. I'm not sure what you mean by *merged*? I ran > it on the shape file I found in the rar you attached to your post. > > I also double checked it by loading it into grass which reported no > topology problems. >> Hugues >> >> "David J. Bakeman" <[email protected]> a écrit : >> >> Which version of QGIS are you using? I just tried your shape file on >> 1.8.0-Lisboa on Linux Fedora 18 and no geometry errors were reported. >> >> On 03/25/2013 07:19 AM, Francois Hugues wrote: >> >> >> Hello, >> >> I received a layer with a lot of "bow tie" polygons which are invalid >> since OGC specifications tell us polygon's boundaries cannot cross itself. I >> first used st_makevalid in postgis which throw me a multipolygon geometry >> composed of two polygons which share one point. But if I look it in qgis and >> try to look for errors (vector/geometry tools/check geometry validity) i >> find errors like "segment 0 of ring 0 of polygon 0 intersects segment 0 of >> ring 0 of polygon 1 at 888252.630181, 6381825.28528". >> >> So I tried somethings outside postgis : i exported my data as exploded >> simple polygons from the original layer to a shp file (like the one attached >> to this message - the example of the turtle - if you would like to reproduce >> the problem). I tried to merge simple polygons with qgis and ran the >> validity checker which throw me the same errors as before. >> >> I definitely think there are no errors with this kind of multipolygon >> even if its components may share some points (here, there is only one, but >> we could find more complicated examples), they seem "topologically" correct >> to me. Am i wrong ? >> >> Hugues. >> >> -- >> >> >> >> Hugues FRANÇOIS >> >> [email protected] >> >> >> >> >> >> _______________________________________________ >> Qgis-user mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> >> _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
