[Qgis-user] Multipolygons with errors ?

[Francois Hugues]

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

hugues.francois at irstea.fr

-------------- next part --------------
A non-text attachment was scrubbed...
Name: the_turtle.rar
Type: application/octet-stream
Size: 2023 bytes
Desc: the_turtle.rar
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20130325/b4cef99d/attachment.obj>