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

[Doug_Newcomb at fws.gov]

Stephen,
      Australia encompasses UTM zones 49 to 56,
http://www.dmap.co.uk/utmworld.htm .  If you tried to use UTM zone 53 for
everything you would have EXTREME distortion at the edges.  You get
noticable distortion when you overlap significantly even into adjoining UTM
zones.

      For an area as large as Australia, you are going to get distortions.
Any map projection involves tradeoffs  in what you are distorting, Area,
Direction, or Distance,
http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html,
http://en.wikipedia.org/wiki/Map_projection

      If you are just looking at raw point to point distance, sticking with
lat/longs and using
http://postgis.refractions.net/documentation/manual-1.3
/ch06.html#id2661454, ST_distance_sphere(point, point)  might be your best
bet.

Doug




Doug Newcomb
USFWS
Raleigh, NC
919-856-4520 ext. 14 doug_newcomb at fws.gov
---------------------------------------------------------------------------------------------------------

The opinions I express are my own and are not representative of the
official policy of the U.S.Fish and Wildlife Service or Dept. of Interior.
Life is too short for undocumented, proprietary data formats.


                                                                           
             Stephen Baillie                                               
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                                       [postgis-users] What coordinate     
             10/01/2008 11:32          system should I use?                
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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

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