[postgis-users] ST_Buffer questions

[Chen, Li [Contractor]]

Thanks Ben,

I will try what you suggested.

We only need to represent mobile cell tower, so geography might be too much for the application. Especially consider there are much more functions for geometry than geography.

Happy Easter:)

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Ben Madin
Sent: Thursday, 1 April 2010 4:42 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] ST_Buffer questions

G'day Li,

I can't help with Q2, but

On 01/04/2010, at 12:28 , Chen, Li [Contractor] wrote:

> Q1.
> ST_Buffer(g1, range) is able to return a geometry within the range of g1.
>  
> So, I define two point using lon/lat (SRID=4326) and range 10km. I want to see whether they cross each other by using ST_Crosses(g1, g2).
> However, I don't know the unit of the range parameter in ST_Buffer(g1,range)  as it is not provide in the docs. So is it km or meters?

The same unit as your Geometry - decimal degrees. Obviously due to the change in the value of this unit at differing latitudes, this is not useful, so a more sensible approach is either to transform your point into a projection using metres, and then use metres

 (off the top of my head it would look like :

select st_buffer(st_transform(g1, appropriate_projection_epsg),10000);

but you should check the docs)

or use the geography type from postgis 1.5? but I haven't tried it yet?

cheers

Ben

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