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<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-GB
style='font-size:10.0pt;font-family:Arial;color:navy'>Thanks for explaining all
this. I think it’s an interesting debate that is often overlooked in
these cases. I think the pointonsurface() function was what I was looking for
this time.<o:p></o:p></span></font></p>
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<p class=MsoNormal style='mso-element:frame'><font size=3 color="#2e3a75"
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>De:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> <st1:PersonName
w:st="on">postgis-users-bounces@postgis.refractions.net</st1:PersonName>
[mailto:<st1:PersonName w:st="on">postgis-users-bounces@postgis.refractions.net</st1:PersonName>]
<b><span style='font-weight:bold'>En nombre de </span></b>Obe, Regina<br>
<b><span style='font-weight:bold'>Enviado el:</span></b> jueves, 14 de junio de
2007 13:52<br>
<b><span style='font-weight:bold'>Para:</span></b> <st1:PersonName w:st="on">PostGIS
Users Discussion</st1:PersonName><br>
<b><span style='font-weight:bold'>Asunto:</span></b> RE: [postgis-users] How is
Centroid(Geometry) calculated?</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>It uses the Geos library to calculate
centroid if Geos is installed. My cursory glance of the Geos code,
looks to me like for polygons and multipolygons</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'> it first takes the area of the
region and then finds the average x,y, z of that encompassing area.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>So yes it is conceivable in a multipolygon
that the centroid may not fall in any of the polygons.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Anyone correct me if I am wrong since I'm
not too familiar with c++ code.</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Anyrate I think the relevant piece of code
in geos library is source\algorithm\CentroidArea.cpp</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Hope that helps,</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Regina</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><font size=2 face=Tahoma><span
lang=EN-US style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>
<st1:PersonName w:st="on">postgis-users-bounces@postgis.refractions.net</st1:PersonName>
[mailto:<st1:PersonName w:st="on">postgis-users-bounces@postgis.refractions.net</st1:PersonName>]
<b><span style='font-weight:bold'>On Behalf Of </span></b>Matt Doughty<br>
<b><span style='font-weight:bold'>Sent:</span></b> Thursday, June 14, 2007 7:03
AM<br>
<b><span style='font-weight:bold'>To:</span></b> postgis-users@postgis.refractions.net<br>
<b><span style='font-weight:bold'>Subject:</span></b> [postgis-users] How is
Centroid(Geometry) calculated?</span></font><span lang=EN-US><o:p></o:p></span></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Hi,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'>Can anybody explain how PostGIS calculates centroids?
I’ve checked the manual but the relevant section doesn’t refer to
an OGC Specification. From what I can understand, it takes an average of the
coordinates of all the vertices of an entry, ignoring whether or not the
centroid falls outside the polygons. Is this right? This differs from other
GIS- GeoMedia, for example, when confronted with a multipolygon entry, always
puts the centroid inside the largest polygon.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'>Can anybody shed some light on this?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'>Cheers,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-GB style='font-size:
10.0pt;font-family:Arial'>Matt<o:p></o:p></span></font></p>
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