[postgis-users] extent of properties within a specified distance

[Andy Anderson]

On May 19, 2008, at 5:01 AM, User Map wrote:
> hi,
> 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:
> SELECT extent(geom),max(Distance('<center of polygon>',geom))as  
> distance
> from tarea
> where within(geom,'<expand(geom,50)from the given geom>')
> and Distance('<center of polygon>',geom)<=50
> 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.

A couple of thoughts, upon which the more experienced may correct or  
elaborate:

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:

	SELECT extent(geom), distance from (SELECT geom, Distance('<center  
of polygon>', geom)) from tarea) as tdistance where distance <= 50.

2) Seems like

	within(geom,'<expand(geom,50)from the given geom>')

is unnecessary, it will always be true. Or do you mean something  
different than the simpler form:

	within(geom, expand(geom,50))

-- Andy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20080520/1b3cfe78/attachment.html>