[postgis-users] Algorithm for Overlaying Areas and Features

[Paul Ramsey]

I'm sorry Renato, computational geometry is beyond the scope of this  
list. You can probably find a good course on it at your local  
university, or if you are more practical minded, the PostGIS source  
code is also available. Or if you just want the answer, load your  
data into PostGIS and apply the recipe provided below! :)

Paul

On 8-Jul-05, at 5:39 PM, Renato F. Salas wrote:

> Hi Paul:
>
> Thank you very much for your comment.
>
> I'm very new to PostGIS and basically I don't have this information  
> stored as postgis tables but as simple points and paths on svg  
> files (xml structured), so I will need to manually do the  
> algorithms to find what is inside.
>
> Could you please explain me what this do internally?.
>
> Why there is a need to do: zips.the_geom && calls.the_geom
> How the contains() function work?
>
> If you have reference such as books or articles about the theory  
> behind this it would be excellent.
>
> Thank you very much for your reply.
>
> Renato
>
>
> Paul Ramsey wrote:
>
>
>> 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
>>   where
>>     zips.the_geom && calls.the_geom
>>   and
>>     contains(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.
>>
>> Paul
>>
>> 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
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
>>>
>>
>>
>>
>
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