[postgis-users] Issue with spatial query (PostGIS, ST_DWithin, SRID, transform)

[Mulone]

Hi Pedro,
Thanks for your reply.

I've tried to re-project everything with 

SELECT osm_id FROM planet_osm_polygon WHERE
ST_DWithin(ST_transform(way,2163),ST_transform(GeomFromText('POINT(53.30124
-6.21804 0)', 4326),2163), 100);


But it doesn't return anything.

That's my scenario: there is just one polygon within 100m, and I'm actually
able to get that object if I enter 84.16 as a distance. Is there a logical
explanation to this? With such a huge distance I would expect postigs to
return everything.


Pedro Doria Meunier wrote:
> 
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> Mulone,
> 
> Openstreetmap uses a Mercator projection.
> When you specify '100' you're actually saying 'give me everything
> within 100 *degrees*'
> 
> You should reproject your data to the desired/applicable projection by
> means of transform();
> 
> HTH,
> 
> Pedro Doria Meunier
> GSM: +351 96 17 20 188
> Skype: pdoriam
>  
> 
> 
> 
> Mulone wrote:
>> [apologies for cross posting]
>>
>> Hello,
>> I'm running some queries on a PostGIS db containing OSM data loaded with
>> osm2pgsql.
>> I want to get the geometries within a certain distance from a point. I've
>> written this query:
>>
>> SELECT osm_id FROM planet_osm_point WHERE
>> ST_DWithin(way,geomfromtext('POINT(53.30124 -6.21804 0)', 4326), 100);
>>
>> where 100 is meant to be 100 meters. This query returns all the objects
>> in
>> the table, and running it with different distances I found a sort of
>> threshold around 84.16, where I start getting less objects.
>>
>> I thought it was an issue related to the system reference, so I tried
>> with
>> this conversion:
>>
>> SELECT osm_id FROM planet_osm_point WHERE
>> ST_DWithin(transform(way,2163),transform(geomfromtext('POINT(53.30124
>> -6.21804 0)', 4326),2163), 5740000);
>>
>> The threshold in this case seems to be about 5740000, which is
> obviously not
>> meters.
>> How can I figure out the units DWithin is working in?
>> Besides, I doubt this is the best way to do what I want, any better
>> ideas?
>>
>> Thanks in advance!
>> Mulone
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