<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=us-ascii">
<META content="MSHTML 6.00.6000.16481" name=GENERATOR></HEAD>
<BODY>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>Dylan,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>So are you saying that with ST_Union you got topological
intersection errors but with ST_MemUnion you didn't? This would be very
intriguing if that were the case</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>since in theory both should yield the same result albeit
slightly differently.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>ST_Union does an array collect and then unions where
as ST_MemUnion keeps on unioning 2 geometries at a time.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>Thanks,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=020223112-06052008><FONT face=Arial
color=#0000ff size=2>Regina</FONT></SPAN></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>Dylan
Lorimer<BR><B>Sent:</B> Monday, May 05, 2008 9:47 PM<BR><B>To:</B>
postgis-users@postgis.refractions.net<BR><B>Subject:</B> [postgis-users] Re:
ST_Difference Perplexes Me!<BR></FONT><BR></DIV>
<DIV></DIV>Wanted to follow-up on this with the SQL that worked in the end.
Firstly...thanks so much Regina! I've no confidence that I ever would have
figured out this sql on my own, and yet looking at it now it makes perfect
sense!<BR><BR>So here it is in case anyone else is interested:<BR><BR>-
vmap_countries contains country boundaries<BR>- published_both_prototype_union
contains dissolved polygons all over the world (millions of them.)<BR><BR>CREATE
TABLE not_meter_or_less_prototype AS <BR>SELECT
ST_Difference(vmap_countries.the_geom, C.the_sum_geom),
vmap_countries.country_na<BR>FROM vmap_countries INNER JOIN (SELECT
ST_MemUnion(published_both_prototype_union.the_geom) As
the_sum_geom,<BR>vmap_countries.country_na<BR> FROM vmap_countries
INNER JOIN published_both_prototype_union ON
ST_Intersects(vmap_countries.the_geom,
published_both_prototype_union.the_geom)<BR>
WHERE vmap_countries.country_na IN ('China', 'India',
'Japan')<BR>
GROUP BY vmap_countries.country_na ) As C<BR>ON vmap_countries.country_na
= C.country_na;<BR><BR>Many many (many) thanks to those on the list that have
helped me out!<BR>Cheers,<BR>dylan<BR></BODY></HTML>
<HTML><BODY><P><hr size=1></P>
<P><STRONG>
The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer.
</STRONG></P></BODY></HTML>
<P><hr size=1></P>
<P><STRONG><font size="2" color="339900"> Help make the earth a greener place. If at all possible resist printing this email and join us in saving paper. </p> <p> </font></STRONG></P>