[postgis-users] Re: Newbie questions: SRIDs, function return values

Bruce Rindahl rindahl at lrcwe.com
Thu May 15 13:11:51 PDT 2008

Walked into that one didn't I :)

For way too much information see:
Skim though the extreme details and read the parts about what's good and 
bad about each type of projection.  This gives you an idea about why 
there are so many ways of mapping information.  This is also the answer 
to original question - "Is a SRID of -1 a problem?"  The answer is yes 
because if you don't know what the projection is,  you can't use the 
resources available to you in PostGIS.

With that in hand you then get to the fun part of the process.  You have 
data in a lat/long projection but you need distances in miles.  Once you 
have the correct projection defined for all you data, transform them 
into a common, appropriate projection and do your distance and intersect 
queries.  The results will be in whatever you want it to be using the 
built in functions available.

Give more details and there will be lots of help on what you need to do.


David Jantzen wrote:
> Bruce: I understand there's a huge conceptual foundation I'm missing
> here.  Is there particular introductory material you would recommend?  I
> should note that my goals are (I think) pretty modest.  I need to map
> lat/longs into neighborhoods, and perform distance queries so that our
> users can restrict their search range to X number of miles from their
> homes.  
> Thanks,
> David

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