[postgis-users] What coordinate system should I use?

[Stephen Baillie]

I've only just come to PostGIS, although I have a bit of PostgreSQL 
experience, and I'm rapidly being overwhelmed by the volume of choices 
available.  So I thought I'd describe my problem in hopes that more 
experienced folks could make some useful suggestions:

I've got ~1.2 million entities with location information in lat/long 
format, and I've been tasked with adding the ability to search based on 
distance (in m or km) from a user-specified point to the existing search 
capabilities.  It looks like to do this with PostGIS I need to add 
another column to my table using |AddGeometryColumn(),| but I'm not sure 
what SRID I should be using.  My data are roughly bounded by latitude 
-10 to -50 and longitude 110-160 (Australia), which puts UTM 53J right 
in the middle, but I'm uncertain as to how much distortion that would 
suffer at the edges.  Would I be best off converting my lat/long pairs 
to WGS84, and building some kind of index on that (typically I'd be 
looking for things in a range around 20km, but could be as much as 
400km)?  Or should I stay with lat/long and use spherical distance 
calculations?  Is there something clever I could do to create 
distance-based indices, or should I just stick with a bounding box on 
lat/long?

Sorry to have so many n00b questions, but any pointers would be appreciated.

Thanks,

Steve.

-- 
Stephen Baillie
Developer
Alliance Software

1/234 Whitehorse Road
Nunawading, VIC 3131
Australia

Ph:  03 9877 9921
Fax: 03 9894 2106