I was having similar issues with unioning country boundaries from the gadm dataset [1], and was thinking how would snaprounding deal with the issue (ie: what brings in, what dangers introduces). What do you think Martin ? Maybe it'd be worth exposing the snapping version more, possibly taking a tolerance explicitly.
[1] http://www.gadm.org/ --strk; () Free GIS & Flash consultant/developer /\ http://strk.keybit.net/services.html On Tue, Apr 13, 2010 at 09:05:07AM -0700, Martin Davis wrote: > PostGIS (actually GEOS, which mirrors JTS) doesn't use a tolerance > during geometry union operations. It's possible that Arc does, which is > why you're seeing a difference. > > What this indicates is that your data is not 100% cleanly noded. You > could try snapping all your data to a small-size grid - that can > sometimes fix problems of this sort. Although it may also introduce > gaps... > > Lee wrote: > > Hi all, First post but have been lurking for a long time. > > > >In the process of migrating some geoprocessing procedures over to our open > >source stack, I came across a difference in results from postgis' st_union > >vs. arcgis' dissolve. From what I can tell these functions should be > >synonymous in theory, despite their naming. (?) > > > >It seems like st_union is not doing a "clean" union, some slivers are > >remaining. > >Original parcel fabric: http://quimby.ca/original.jpg > >Dissolved by ArcGIS: http://quimby.ca/dissolved_by_arcgis.jpg > >st_union by postgis: http://quimby.ca/dissolved_by_postgis.jpg > > > >The input to both functions was the same table in postgis. I am defining > >the geometry simply as select > >st_union(current_assessment_parcel.the_geom) AS the_geom > > > >Postgresql 8.4.1 > >Postgis 1.4 > > > >Is this simply a difference in tolerance? or am I seeing something else > >here. > > > >Thanks, > >Lee. _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
