[postgis-users] Algorithm for Overlaying Areas and Features

Paul Ramsey pramsey at refractions.net
Fri Jul 8 16:58:23 PDT 2005

This is PostGIS land, so the answer looks like this (with variation  
for whatever your column names are):

   select count(*) as num_calls, zips.zipcode
   from zips, calls
     zips.the_geom && calls.the_geom
   group by zipcode;

The answer is formulated differently for other tools, but as you  
note, it is well understood and oft-repeated.


On 8-Jul-05, at 4:22 PM, Renato Salas wrote:

> Hi group:
> I'm curious about what is the underlying algorithm used to find out  
> if a geo-feature is within an area.
> For instance, I have a dataset with emergency calls and another  
> with zip areas. Then I want to know how many calls are in each area.
> I know I need to tag each feature with the zip area code it belongs  
> to and then sum the number of calls for each area.
> My question is, how do I determine in which area a particular call  
> belong in an efficiente manner. Do I have to step in each area to  
> find which features are inside its boundaries?. Do I first need to  
> sort the features by Lat/Lon so that I know the boundaries of my  
> features, thus reducing the number of candidates areas?.
> I'm pretty sure this issue is well know and solved on the GIS  
> community so please illuminate me since I don't want to reinvent  
> the wheel.
> Thanks all in advance for your comments,
> Regards,
> Renato
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