> Good people; > > I have this odd problem that I hope others may clarify for me. > > I have a big ArcInfo polygon coverage produced by a sequence of spatial > unions. One of the component polygon coverages was brutalized somewhere > along the way and it has some polygons with triangular and bow-tie > inclusions. > > Nevertheless, the coverage is clean and sober as far as ArcInfo is > concerned. > > If you all can picture in your minds a bow-tie polygon |><| where the > rightmost | of the bowtie is actually the edge of an enclosing polygon, kind > of like: > > | > |><| > | > > (I sure hope that works out visually...) > > OK, when I use ogr2ogr to import this coverage into postgis 1.3.1 / > geos-3.0.0rc4, I see the following: > > > > the containing polygon fails at st_isValid() > the st_area() of the containing polygon appears to include the area of the > bow-tie polygon, ie it is larger than the area copied over from the ArcInfo > coverage by about the same amount as the area of the bow-tie polygon When I > look at this little mess with OpenJump, sure enough the QA routines find the > bow-tie and complain. > > The long and the short of this is that PostGIS returns slightly more area > than ArcInfo does for the same big ugly polygon coverage, because of the > apparent loss of these bow ties. > > What I'm not sure about is what exactly is wrong here. Is the bow-tie > shape itself illegal? Or is it the fact that both sides of the bow-tie > belong to the same polygon? Or something else I'm missing? > > Thanks for any light anyone can shed on this. > >
Hi, I will only give a partial answer: yes, a bowtie polygon is invalid for PostGIS (as stated in the OGC SFSQL documentation, polygon's boundary cannot cross itself). If you want to represent this with valid polygons, you will need 2 triangles touching at one point. HTH nicolas _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
