[postgis-users] Path to closest neighbor

[Nalin C]

Hi Steve,

I applied the query for several accident places which are enough far away to
generate different results, but produced the same result finally.

 


Stephen Woodbridge wrote:
> 
> Well, you might want to look into this query then and see if you can 
> find the issue. If you have a reasonable number of hospitals then just 
> list them all like:
> 
> select name,
>         astext(the_geom),
>         distance_sphere(setsrid(makepoint(x,y), 4326), the_geom) as dist
>    from hospitals
>   order by
>         distance_sphere(setsrid(makepoint(x,y), 4326), the_geom) asc;
> 
> Distance should be in meters. You might want to validate the the 
> hospitals are entered at the correct positions. Computing the distance 
> is pretty basic stuff, it is probably a problem with the data.
> 
> -Steve
> 
> Nalin C wrote:
>> Ya, it's near the southern tip of India, a point in Sri Lanka. x, y has
>> been
>> given correct. I used 4326 as the SRID. 
>> 
>> 
>> 
>> Stephen Woodbridge wrote:
>>> You need to read up on pgRouting and probably join the pgRouting list
>>>
>>> http://pgrouting.postlbs.org/
>>> http://lists.postlbs.org/mailman/listinfo/pgrouting-users
>>>
>>> -Steve
>>>
>>> Nalin C wrote:
>>>> Also, can you please suggest me the functions that can be used for
>>>> points
>>>> 2
>>>> and 3 ?  I'm totally a newbie to postgis. 
>>>>
>>>> In point 2, it would be great if the path can start from in front of
>>>> the
>>>> hospital, instead a vertex in roads table.
>>>>
>>>>
>>>>
>>>> Nalin C wrote:
>>>>> Hi Steve,
>>>>>
>>>>> Thank you for the quick reply.
>>>>>
>>>>> I observed that the results returned by :
>>>>>
>>>>> select name, the_geom from hospitals order by
>>>>> distance_sphere(setsrid(makepoint(79.877778, 6.914722), 4326),
>>>>> the_geom)
>>>>> asc limit 3; 
>>>>>
>>>>> are not the closest hospitals to the given accident point. I'm not
>>>>> sure
>>>>> what has gone wrong.
>>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Nalin
>>>>>
>>>>>
>>>>>
>>>>> Stephen Woodbridge wrote:
>>>>>> Nalin C wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> I have two tables for roads and hospitals. I'm looking for suitable
>>>>>>> pgrouting function(s) for following scenario.
>>>>>>>
>>>>>>> Given the accident point (in point geometry), finding the least
>>>>>>> distant
>>>>>>> hospital and draw the path.
>>>>>>>
>>>>>>> Following functionality is needed.
>>>>>>>
>>>>>>> 1. One end of the drawn path needs to start from in front of the
>>>>>>> closest
>>>>>>> hospital (which is in hospitals table).
>>>>>>> 2. Path needs to stop from the accident point on road.
>>>>>>>
>>>>>>> I'm new to pgrouting and appreciate your help.
>>>>>> You have a few discrete tasks. Assuming you have the location of the 
>>>>>> accident:
>>>>>>
>>>>>> 1) Select say the three closest hospitals based on straight line 
>>>>>> distance. (x,y) is the location of the accident.
>>>>>>
>>>>>> select name, the_geom from hospitals where order by 
>>>>>> distance_sphere(setsrid(makepoint(x,y), 4326), the_geom) asc limit 3;
>>>>>>
>>>>>> 2) then use pgRouting to use the Hospital location as start and the 
>>>>>> accident as end and comput the routes
>>>>>>
>>>>>> 3) compare the routed distances and select the shortest.
>>>>>>
>>>>>> -Steve W
>>>>>> _______________________________________________
>>>>>> postgis-users mailing list
>>>>>> postgis-users at postgis.refractions.net
>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>>
>>>>>>
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>> 
> 
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