[postgis-users] How to use accurate SRID ?

[nguyen liem]

Hi all,

I stored all my geometry in SRID 4326. I would like to calculate the
distance between 2 points in metter.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
I see an example about ST_Distance_Sphere here:
http://postgis.refractions.net/docs/ST_Distance_Sphere.html

SELECT round(CAST(ST_Distance_Sphere(ST_Centroid(the_geom),
ST_GeomFromText('POINT(-118 38)',4326)) As numeric),2) As dist_meters,
round(CAST(ST_Distance(ST_Transform(ST_Centroid(the_geom),32611),
		ST_Transform(ST_GeomFromText('POINT(-118 38)', 4326),32611)) As
numeric),2) As dist_utm11_meters,
round(CAST(ST_Distance(ST_Centroid(the_geom),
ST_GeomFromText('POINT(-118 38)', 4326)) As numeric),5) As
dist_degrees,
round(CAST(ST_Distance(ST_Transform(the_geom,32611),
		ST_Transform(ST_GeomFromText('POINT(-118 38)', 4326),32611)) As
numeric),2) As min_dist_line_point_meters
FROM
	(SELECT ST_GeomFromText('LINESTRING(-118.584 38.374,-118.583 38.5)',
4326) As the_geom) as foo;
	 dist_meters | dist_utm11_meters | dist_degrees | min_dist_line_point_meters
	-------------+-------------------+--------------+----------------------------
		70424.47 |         *70438.00* |      0.72900 |                   65871.18


SRID 32611: the distance is *70438.00* metters.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
If i changed SRID from 32611 to 900913

 dist_meters | dist_utm11_meters | dist_degrees | min_dist_line_point_meters
	-------------+-------------------+--------------+----------------------------
		70424.47 |         *89738.89* |      0.72900 |                   83857.70


SRID 32611: the distance is *89738.89* metters


I don't know what is the different between SRID 32611 and SRID 900913 ?
I should use 32611 or 900913 to calculate the distance in metter ?
Could i calculate distance by ST_Distance_Sphere for geometry with SRID 4326
?
Whats the different between ST_Distance_Sphere(SRID 4326), ST_Distance(SRID
32611), ST_Distance (SRID 900913) ?

Cheers.

-- 
*Liem Nguyen (DBA)

AXON ACTIVE VIETNAM*
*www.axonactive.vn***

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Chi Minh City, Vietnam
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