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<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>Brian,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>I guess I should ask first what were you expecting to
happen?</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>The concave hull always returns a single polygon. It
should never return a multipolygon. So given that constraint, I don't see
how it could avoid splitting the edge of your example.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>Note the union of your polygon is a
multipolygon</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>SELECT ST_Astext(ST_Union('POLYGON((13 0,13 20,23 20,23 0, 13
0))'::geometry, 'POLYGON((23 20, 23 28, 30 28,30 20, 23
20))'::geometry));</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>That said any collection of polygons when unioned that does
not result in a single polygon will have edges split when forming the concave
hull.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN><SPAN class=466182800-14052012><FONT
color=#0000ff size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>Hope that helps,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial>Regina</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial><A
href="http://www.postgis.us">http://www.postgis.us</A></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=466182800-14052012><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV><BR>
<BLOCKQUOTE
style="BORDER-LEFT: #0000ff 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px"
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<FONT size=2 face=Tahoma><B>From:</B>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <B>On Behalf Of
</B>Brian Sanjeewa Rupasinghe<BR><B>Sent:</B> Sunday, May 13, 2012 2:27
PM<BR><B>To:</B> postgis-users@postgis.refractions.net<BR><B>Subject:</B>
[postgis-users] ConcaveHull generation<BR></FONT><BR></DIV>
<DIV></DIV>Hi,
<DIV><BR></DIV>
<DIV>I have two polygons with following coordinates:</DIV>
<DIV><BR></DIV>
<DIV>poly1: (13,0 13,20 23,20 23,0 13,0)</DIV>
<DIV><BR></DIV>
<DIV>poly 2: (23,20 23,28 30,28 30,20 23,20)</DIV>
<DIV><BR></DIV>
<DIV>Once generated concaveHull, i end up with the polygon having following
coordinates:</DIV>
<DIV><BR></DIV>
<DIV>
<P class=MsoNormal>"POLYGON((13 3e-016,13 5.64256204109163,13
11.2851240821833,13 16.9276861232749,13 20,17.4061007032357 20,23
27.049761125177,23 28,28.6425620410916 28,30 28,30 22.3574379589084,30 20,23
14.674220666202,23 9.34844133240395,23 4.02266199860593,23
3e-016,17.3574379589084 3e-016,13 3e-016))"</P>
<P class=MsoNormal><BR></P>
<P class=MsoNormal>When investigating the coordinate list, it is observed that
some existing edges are split. [Eg. edge connecting 13,0 and 13,20]. Why is
taht?</P>
<P class=MsoNormal><BR></P>
<P class=MsoNormal>Here is the SQL code i used</P>
<P class=MsoNormal><BR></P>
<P class=MsoNormal></P>
<P class=MsoNormal>SELECT ST_AsText
(ST_cleangeometry((ST_ConcaveHull(ST_Collect(geom), 0.99))))</P>
<P class=MsoNormal>FROM concavepoly</P>
<P class=MsoNormal>WHERE cluster_id = 'VC-2';</P>
<P class=MsoNormal><BR></P>
<P class=MsoNormal>Cheers, Brian</P>
<P></P></DIV></BLOCKQUOTE></BODY></HTML>