[postgis-users] within function

[Mark Cave-Ayland]

On Tue, 2007-08-07 at 09:06 +0100, Dave Potts wrote:
> I have a table with a list of places described as Geometry type POINT 
> and a table loaded from a shape file,  which takes the form of target 
> (ie several inscribed circles), each is encoded as a MULTIPOLYGON.  I am 
> attempting to discover which point lie within within these circle.
> 
> I had assumed that something like
> 
> select place_name,gid,id,gridcode,area(the_geom) , 
> within(the_geom,os_ngr) from shptbl,place_table wherein (the_geom,os_ngr) ;
> 
> would return every geometry ie os_ngr which is within the geometry  
> boundries of the_geom.
> 
> Where place_name,os_ngr come from place_table and the_geom comes from 
> the table shptbl.
> 
> Both geometry have the same sri, no errors are listed in the postgres 
> log files.
> I have assume that within covers data types of different types (ie POINT 
> and MULTIPOLYGON)
> 
> Dave.


Hi Dave,

We need a bit more information to help you here; what query results are
you seeing and what query results are you expecting to see? The only
thing I can see is that Within(A, B) returns whether geometry A is
within geometry B rather than vice-versa, so you might need to swap your
parameters to Within().

You may like to try the following (if you have a GiST index on your
the_geom and os_ngr columns then the && will ensure that it can be used
to calculate the much faster bounding box overlap rather than the
complex and slower Within() function):


select place_name, gid, id, gridcode, area(the_geom), within(os_ngr,
the_geom) from shptbl, place_table where
os_ngr && the_geom;


HTH,

Mark.

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ILande - Open Source Consultancy
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