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<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>How have you failed? Did you get an error or some
unexpected result.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>ST_Difference takes as input 2 geometries -
ST_Difference(geom1, geom2) and returns that portion of geom1 that is not
in geom2.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>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.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT><FONT face=Arial color=#0000ff
size=2><SPAN class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>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</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>SELECT ST_Difference(l.the_geom,
b.the_geom)</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>FROM parcels l INNER JOIN buildings b ON
ST_Intersects(l.the_geom, b.the_geom)</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>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.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT><FONT face=Arial color=#0000ff
size=2><SPAN class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>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.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>Hope that helps,</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007>Regina</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=095542920-22102007></SPAN></FONT> </DIV><BR>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <B>On Behalf Of
</B>Richard Heimann [C]<BR><B>Sent:</B> Monday, October 22, 2007 3:47
PM<BR><B>To:</B> postgis-users@postgis.refractions.net<BR><B>Subject:</B>
[postgis-users] ST_Difference Syntax<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV class=Section1>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">PostGIS users,</SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"></SPAN></FONT> </P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">I have failed at constructing the
ST_Differnece operation. Can someone expose the proper syntax?
Thanks</SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"></SPAN></FONT> </P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">Richard</SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"></SPAN></FONT> </P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"></SPAN></FONT> </P>
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