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

[Paul Ramsey]

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
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>>>
>>
>>
>
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