[postgis-users] st_union with few overlapping elements

David Epstein davideps at umich.edu
Sat Feb 2 12:16:44 PST 2008


I am new to postGIS and am using the st_union function for the first
time. I have a table with 10K polygons and another table with about 100
polygons. The second layer contains elements missing from the first, but
there may be some overlap along the edges where they fit together. They
have different attribute columns.

I want to create a new polygon table with ALL of this data and with the
unique key necessary for loading in QGIS. I've seen some relevant
examples with intersection, but have not been able to change them to do
what I want yet. Pasted below is the code that I tried, which produced a
file with 700k rows!!! I could not load it in QGIS so I am not sure what
it represents, perhaps all the points? Do I need to pre-define the table
that receives the attributes (I hope not, because there are many!)

ST_UNION(h.the_geom, m.the_geom) AS union_geom
have_plots AS h,
missing_plots AS m;

Any help would be appreciated! 

thank you,

PS: I am currently executing queries in PGADMIN3 to make mass
manipulations to the data & using QGIS to visualize, geo-reference, and
move specific vertices. Does anyone highly recommend a different
combination of tools?

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