[postgis-users] ST_Buffer questions

[Ben Madin]

To look up an appropriate projection, you have a number of options:

Easy : http://www.spatialreference.org/

Sort of easy : SELECT srid, srtext FROM spatial_ref_sys WHERE srtext LIKE '%GDA94%'; -- To find something containing Geocentric Datum of Australia 1994 

But Australia is a big country, so it depends where in Australia you are - and how accurate you want to be. 

In Western Australia you might get away with just using epsg:28350, but you could choose better for Macquarie Island 28356?

If you are going globally you might want to just use normal UTM Zones (WGS84). The South ones all start with 327_ _... ie Western Australia would be 32750. The North ones start with 326_ _.

Good luck - look at the website above.

cheers

Ben




On 15/04/2010, at 14:15 , Chen, Li [Contractor] wrote:

> Hi,
> 
> Do you might know a proper "appropriate_projection_epsg"? I am in Australia but I might need to use data from the whole world.
> 
> Also, I was not able to find a SRID for ECEF reference system, does anyone know?
> 
> Thanks,
> 
> Li
> 
> -----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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