[postgis-users] st_union

[Lee]

 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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