[postgis-users] st_union

[strk]

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
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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.