[postgis-users] ST_Difference Syntax

[Obe, Regina]

How have you failed?  Did you get an error or some unexpected result.
 
ST_Difference takes as input 2 geometries - ST_Difference(geom1, geom2)
and returns that portion of geom1 that is not in geom2.
 
I generally use it in conjunction with intersection.  If 2 geometries
are Disjoint - do not intersect then ST_Difference returns just geom1 I
believe which is not terribly interesting and a bit of a waste fo
processing speed since there are easier ways to return that.
 
So you would use it something like this - for example if you wanted to
know that portion of a land parcel that is not built on, then you would
do something like
 
SELECT ST_Difference(l.the_geom, b.the_geom)
FROM parcels l INNER JOIN buildings b ON ST_Intersects(l.the_geom,
b.the_geom)
 
 
Note here I am making the wrong assumption that only one building lies
on a parcel - so this example is a bit flawed (but simpler than my real
query), but hopefully will give you an idea of how it is used.
 
 
Note: There is the less popular ST_SymDifference(geom1, geom2) that
returns that portion of geom1 + geom2 that is not shared.  Thus it is
called Symmetric Difference since the order of the arguments does not
matter.
 
Hope that helps,
Regina
 

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Richard Heimann [C]
Sent: Monday, October 22, 2007 3:47 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] ST_Difference Syntax



PostGIS users,

 

I have failed at constructing the ST_Differnece operation. Can someone
expose the proper syntax? Thanks

 

Richard

 

 

 




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