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<P>hi there,</P>
<P>thanks for the immediate reply. i have tried what you have suggested, and its now taking no more longer time spans :) but i dont know how it is calculating the distance, because the extent returned by the query when used in the mapfile for mapserver with a scalebar, it is giving much larger area of about 800m in total, which should be about 100 m in actual as defined in the query.</P>
<P>i have this query</P>
<P>________________________________________________________________________</P>
<P>SELECT ST_Extent(geom) as ext, count (*),Max(Distance(geom,GeomFromText('POINT(530525.056494402 191742.597993273)',27700))) as max_dist</P>
<P>FROM tarea</P>
<P>WHERE ST_DWithin(geom,GeomFromText('POINT(530525.056494402 191742.597993273)',27700), 50)</P>
<P>__________________________________________________________________________</P>
<P>POINT(530525.056494402 191742.597993273) is got from AsText(Centroid(geom))</P>
<P>i have OSGB36 data, and srid= 27700, which i have set in the query. </P>
<P>the extent returned is much larger. but the max_dist is returning 49.5, which is fine.</P>
<P>do you know why it is not matching the scalebar in the mapserver. does the mapserver scalebar works differently then the Distance query.</P>
<P>any help will be appriciated</P>
<P>regards</P></FONT><BR><BR>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">----- Original Message ----<BR>From: "Obe, Regina" <robe.dnd@cityofboston.gov><BR>To: PostGIS Users Discussion <postgis-users@postgis.refractions.net><BR>Sent: Wednesday, May 21, 2008 11:38:57 AM<BR>Subject: RE: [postgis-users] extent of properties within a specified distance<BR><BR>
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<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>Have you tried using ST_DWithin(...) ?</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>- even with Within (neither Distance nor Within use spatial indexes so you are essentially always doing a table scan. You would still need an && for those to make them use indexes. So try:</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008>SELECT ST_Extent(geom) as ext, Max(ST_Distance(geom, '<center of polygon>')) as max_dist</SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008>FROM tarea<BR>WHERE ST_DWithin(geom,'<center of polygon>', 50);</SPAN></DIV>
<DIV><SPAN class=187402510-21052008></SPAN> </DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>If you are using an older version of Postgis that doesn't have the ST_ (e.g. pre 1.3)</FONT></SPAN></DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>Do the following instead</FONT></SPAN></DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT> </DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008>SELECT Extent(geom) as ext, Max(Distance(geom, '<center of polygon>')) as max_dist</SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=187402510-21052008>FROM tarea<BR>WHERE geom && Expand('<center of polygon>',50) AND Distance(geom, '<center of polygon>') < 50;</SPAN></DIV>
<DIV><SPAN class=187402510-21052008></SPAN> </DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>Hope that helps,</FONT></SPAN></DIV>
<DIV><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2>Regina</FONT></SPAN></DIV>
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<DIV dir=ltr align=left><SPAN class=187402510-21052008><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV><FONT face=Arial color=#0000ff size=2></FONT><BR>
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<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>User Map<BR><B>Sent:</B> Wednesday, May 21, 2008 5:19 AM<BR><B>To:</B> PostGIS Users Discussion<BR><B>Subject:</B> Re: [postgis-users] extent of properties within a specified distance<BR></FONT><BR></DIV>
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<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">
<P>hi, </P>
<P>thanks, but the problem is still there, the time Distance() query is taking. the concept behind using within() is only to minimize the number of time the distance function executes. it works ok without using within() also, but the time is still much longer. is there any way to minimize the time it is taking to perform distance(), or any other way to do the same task without using distance().</P>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif"> </DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">regards.<BR><BR>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: times new roman, new york, times, serif">----- Original Message ----<BR>From: Andy Anderson <aanderson@amherst.edu><BR>To: PostGIS Users Discussion <postgis-users@postgis.refractions.net><BR>Sent: Tuesday, May 20, 2008 10:09:27 PM<BR>Subject: Re: [postgis-users] extent of properties within a specified distance<BR><BR>
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<DIV><FONT class=Apple-style-span color=#000000>On May 19, 2008, at 5:01 AM, User Map wrote:</FONT></DIV>
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<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>hi,</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>thanks for reply. i think i have to clarify my problem a bit more. i want to get the records from my table within a specified distance for e.g 50 meters. For this i have to calculate the extent first to get the image in mapserver. what i have is just a polygon geom, which area as you know can vary from property to property. i dont want to take into account the area/distance the property holds, but a center point for that polygon, so to calculate the distance equally on each side of that point, and then calculate the extent of those records, for the image to be generated. what i have done uptil now is:</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>SELECT extent(geom),max(Distance('<center of polygon>',geom))as distance</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>from tarea</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>where within(geom,'<expand(geom,50)from the given geom>')</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>and Distance('<center of polygon>',geom)<=50</FONT></P>
<P class=MsoNormal style="MARGIN: 5pt 0in"><FONT class=Apple-style-span color=#000000>but this query is taking about 25-26 mins which is quite a large time span. and also when i get the results and on showing through the map server, the scale bar is not matching, i mean it has to show a 100 meter bar scale, but, it is showing more then 100 meters on the scalebar, about 192 or so.</FONT></P></DIV></DIV></DIV></DIV></SPAN></BLOCKQUOTE><FONT class=Apple-style-span color=#000000><BR></FONT></DIV>
<DIV><FONT class=Apple-style-span color=#000000>A couple of thoughts, upon which the more experienced may correct or elaborate:</FONT></DIV>
<DIV><BR></DIV>
<DIV><FONT class=Apple-style-span color=#000000><FONT class=Apple-style-span color=#000000>1) It looks to me like you are performing a duplicate calculation. I'm not sure if Postgres is smart enough to avoid this by itself, but just in case you should take advantage of a subexpression to perform the calculation once for each geom:</FONT></FONT></DIV>
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<DIV><FONT class=Apple-style-span color=#000000><SPAN class=Apple-tab-span style="WHITE-SPACE: pre"></SPAN>SELECT extent(geom), distance from (SELECT geom, Distance('<center of polygon>', geom)) from tarea) as tdistance where distance <= 50.</FONT></DIV>
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<DIV><FONT class=Apple-style-span color=#000000>2) Seems like</FONT></DIV>
<DIV><FONT class=Apple-style-span color=#000000><BR></FONT></DIV>
<DIV><SPAN class=Apple-tab-span style="WHITE-SPACE: pre"><FONT class=Apple-style-span color=#000000></FONT></SPAN><FONT class=Apple-style-span color=#000000>within(geom,'<expand(geom,50)from the given geom>')</FONT></DIV>
<DIV><FONT class=Apple-style-span color=#000000><BR></FONT></DIV>
<DIV><FONT class=Apple-style-span color=#000000>is unnecessary, it will always be true. Or do you mean something different than the simpler form:</FONT><FONT class=Apple-style-span color=#000000></FONT></DIV>
<DIV><FONT class=Apple-style-span color=#000000><BR></FONT></DIV>
<DIV><SPAN class=Apple-tab-span style="WHITE-SPACE: pre"><FONT class=Apple-style-span color=#000000></FONT></SPAN><FONT class=Apple-style-span color=#000000>within(geom, expand(geom,50))</FONT></DIV>
<DIV><BR></DIV>
<DIV>-- Andy</DIV>
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