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

[Mulone]

I just realised that... Thanks.

But then again, what SRID am I supposed to use to get meters?
Sorry but I couldn't find any proper documentation on this apart from blog
posts.

For example, the query:

SELECT distance(
transform( GeomFromEWKT('SRID=4326;POINT( -6.195  53.283 0)'), 32661 ),
transform( way, 32661 )
),*
FROM planet_osm_point
WHERE osm_id = 47204752

seems to return meters, but how can I figure out the query precision and
reliability?

Thanks again,
Mulone


Paul Ramsey-3 wrote:
> 
> It means you should be using POINT(-6.195 53.283 0), you are reversing
> your coordinates for the lonlat point.
> 
> P
> 
> On Fri, Jul 24, 2009 at 9:48 AM, Mulone<andrea.ballatore at ucd.ie> wrote:
>>
>> If I execute:
>>
>> SELECT distance(GeomFromEWKT('SRID=4326;POINT(53.283 -6.195 0)'), way),*
>> FROM planet_osm_point WHERE osm_id = 47204752
>>
>> the expected distance is about 100-150 meters, and I get
>> 84.1155775105245, which is in decimal degrees, but what does that mean?
>> Degrees from what?
>>
>> If I re-project everything in another SRID, I get the following values,
>> which aren't less mysterious to me:
>> SRID: 32662 ===> distance: 9363703.25625372
>> SRID: 26986 ====> distance: 9366636.40006539
>>
>> Any ideas?
>>
>> Mulone
>>
>>
>>
>> Mulone wrote:
>>>
>>> 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:
>>>>
>>>> -----BEGIN PGP SIGNED MESSAGE-----
>>>> Hash: SHA1
>>>>
>>>> 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
>>>> -----BEGIN PGP SIGNATURE-----
>>>> Version: GnuPG v1.4.7 (GNU/Linux)
>>>> Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org
>>>>
>>>> iD8DBQFKaK5G2FH5GXCfxAsRAiNvAJwND0pmRWNaODnJ0foW9uraaS1YnACfbdIX
>>>> +de0ZbKmALUpjlFbzbO/rnk=
>>>> =eWY7
>>>> -----END PGP SIGNATURE-----
>>>>
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>>>
>>>
>>
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