[postgis-users] Problem with ST_SymDifference function

[n.kolios]

Hi Leo, thanks for your reply, these are two MULTIPOLYGON tables that I am
trying to compare. 


Paragon Corporation-2 wrote:
> 
> Nikos,
> 
> What kind of geometries are you talking about? In some cases the
> ST_Symdifference will look the same as the original depending on the
> dimensionality of the input geometries.
> 
> If you have two sets of polygons -- you'll see a difference but a line and
> polygon you may or may not because if the line intersects the polygon --
> the
> symetric difference will look like pretty much the union of the polygon
> with
> the line (if the line is completely contained by the polygon -- it will
> look
> like just the polygon (the union of the 2)
> 
> 
> Leo 
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Kolios
> N.
> Sent: Monday, August 03, 2009 4:58 AM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] Problem with ST_SymDifference function
> 
> Hi all,
> 
> I am using an ST_SymDifference query in order to visualize changes between
> two polygon tables.
> 
> The result, however does not look correct. It includes the entire
> geometries
> of both tables rather than the portions that do not intersect.
> 
> My query is the following:
> 
> CREATE TABLE bu_delta as
> SELECT
> ST_SymDifference(t.the_geom, b.the_geom) AS difference_geom FROM
> bu_benchmark AS b, bu_test AS t WHERE ST_Intersects(t.the_geom,
> b.the_geom);
> 
> (and then add gid)
> 
> Has anyone ever done something similar?
> Thanks in advance,
> Nikos
> 
> 
> PS1: The geometries of my tables are such that one feature from one layer
> intersects with multiple features from the other layer.
> PS2: When I run this query for one isolated polygon from each table the
> result is OK.
> PS3: I also tried the ST_Difference function with no success.
> 
> A picture might help:
> http://www.nabble.com/attachment/24716126/0/example_difference.TIF
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