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
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