# [postgis-users] Antwort: Re: Projection(s) for global point-and-radius searches

Chen, Li [Contractor] Li.Chen at andrew.com
Wed Jul 7 23:53:52 PDT 2010

```I think you can try to covert lat/longs to meters, but this is highly inaccurate, especially in places where the latitude is greater than 75.
This problem troubles us as well

From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Stefan Priess
Sent: Thursday, 8 July 2010 4:52 PM
To: PostGIS Users Discussion
Subject: [postgis-users] Antwort: Re: Projection(s) for global point-and-radius searches

Hey Paul,

i need the ST_Expand() function with a radius/expand of 10 Meters, how i will do that?
The Point to expand is in SRID 4326 .

Thanks

Stefan

Paul Ramsey <pramsey at opengeo.org>
Gesendet von: postgis-users-bounces at postgis.refractions.net

07.07.2010 23:13
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Re: [postgis-users] Projection(s) for global point-and-radius        searches

No, because the functions on Geometry(4326) don't return metric answers.

SELECT
ST_Distance('SRID=4326;POINT(0 0)'::geometry, 'SRID=4326;POINT(1
1)'::geometry) as geom,
ST_Distance('POINT(0 0)'::geography, 'POINT(1 1)'::geography) as geog;

Geography understands that it's on a sphere and does the Right Thing.
Geometry(4326) still operates on a cartesian plane, it just has
"units" of degrees (which is pretty nonsensical for any kind of
measurement calculation).

P

On Wed, Jul 7, 2010 at 2:03 PM, David Jantzen <djantzen at ql2.com> wrote:
> Ok, interesting.  Is it equivalent then to using a GEOMETRY type with SRID 4326?
>
> On Jul 7, 2010, at 12:25 PM, Paul Ramsey wrote:
>
>> On Wed, Jul 7, 2010 at 12:25 PM, Paul Ramsey <pramsey at opengeo.org> wrote:
>>> ST_Buffer(geography, float8) returns geography
>>> ST_Intersection(geography, geography) returns geography
>>
>> These last two actually *do* carry out projections under the covers,
>> so watch out.
>>
>> P
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