[postgis-users] st_union

[Martin Davis]

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.
>   
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022