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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>ST_Centroid doesn’t require a defined SRID, so in theory ST_Centroid(geography_column::geometry) should work fine.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The biggest problem with such a cast is you could pretty easily come up with strange centroid results because your are treating an angular coordinate system as a planar one. It may not be a problem, however, if your geometries aren’t spanning too large a geographic area.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Alternatively (and It would be a little messy), you could do a SELECT CASE with constraints on longitudinal ranges to do individual casts as UTM (and if you are working in polar regions, also have cases for latitudes for the polar stereographics… ). It would be a quasi-rigourous approach, but you could transform them back after the centroid is calculated.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Best,<br>Steve<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><!--[if gte vml 1]><v:shapetype id="_x0000_t75" coordsize="21600,21600" o:spt="75" o:preferrelative="t" path="m@4@5l@4@11@9@11@9@5xe" filled="f" stroked="f">
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</v:shape><![endif]--><![if !vml]><img width=90 height=122 src="cid:image001.png@01CDD3CF.96BB4A00" align=left hspace=12 alt="http://www.clemetparks.com/images/esig/cmp-ms-90x122.png" v:shapes="cmp-ms-90x122.png"><![endif]><span style='font-size:14.0pt;font-family:"Arial","sans-serif";color:#006C56'>Stephen Mather<br></span><span style='font-size:11.0pt;font-family:"Arial","sans-serif";color:#006C56'>Geographic Information Systems (GIS) Manager<br></span><span style='font-size:9.0pt;font-family:"Arial","sans-serif";color:#006C56'>(216) 635-3243<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.0pt;font-family:"Arial","sans-serif";color:#006C56'>svm@clevelandmetroparks.com<br></span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><a href="http://www.clemetparks.com/"><span style='font-size:9.0pt;font-family:"Arial","sans-serif";color:blue'>clevelandmetroparks.com</span></a><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> postgis-users-bounces@lists.osgeo.org [mailto:postgis-users-bounces@lists.osgeo.org] <b>On Behalf Of </b>Ed Linde<br><b>Sent:</b> Thursday, December 06, 2012 4:21 PM<br><b>To:</b> PostGIS Users Discussion<br><b>Subject:</b> Re: [postgis-users] Computing a centroid on GEOGRAPHY objects<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Yeah, but doesn't that need a spatial reference ID? Or would it automatically know what to do?<o:p></o:p></p><div><p class=MsoNormal style='margin-bottom:12.0pt'><o:p> </o:p></p><div><p class=MsoNormal>On Thu, Dec 6, 2012 at 9:40 PM, Michal Zimmermann <<a href="mailto:zimmicz@gmail.com" target="_blank">zimmicz@gmail.com</a>> wrote:<o:p></o:p></p><p class=MsoNormal>You can try to cast geography to geometry with<br>ST_Centroid(geography_column::geometry).<o:p></o:p></p><div><div><p class=MsoNormal style='margin-bottom:12.0pt'><br>On 12/6/12, Ed Linde <<a href="mailto:edolinde@gmail.com">edolinde@gmail.com</a>> wrote:<br>> Hi All,<br>> I have a huge bunch of scattered lat/long coordinates stored as GEOGRAPHY<br>> type. I would now like to cluster some of them and<br>> compute their centroids.<br>> I know there is a centroid computation function in Postgis, but it seems to<br>> only work on a geometry type.<br>> The lat/long coordinates are spread all across the globe and hence I cannot<br>> convert it to local UTM formats because there is a discrepancy when going<br>> across different zones!<br>> Any workarounds?<br>><br>> Thanks,<br>> Ed<br>><br><br><o:p></o:p></p></div></div><p class=MsoNormal><span class=hoenzb><span style='color:#888888'>--</span></span><span style='color:#888888'><br><span class=hoenzb>Michal Zimmermann (zimmi)</span><br><span class=hoenzb>WWW: <a href="http://www.zimmi.cz" target="_blank">http://www.zimmi.cz</a></span><br><span class=hoenzb>_______________________________________________</span><br><span class=hoenzb>postgis-users mailing list</span><br><span class=hoenzb><a href="mailto:postgis-users@lists.osgeo.org">postgis-users@lists.osgeo.org</a></span><br><span class=hoenzb><a href="http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users" target="_blank">http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users</a></span></span><o:p></o:p></p></div><p class=MsoNormal><o:p> </o:p></p></div></div></body></html>