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

[Burgholzer,Robert]

Bruce,
Your point is well taken, but I suppose that I misrepresented my real
interest.  While it is nice to provide functions that will *just work*
for people who don't know what they are doing, this type of function is
also quite useful for people who are writing software, and looking for a
way to increase the robustness of the application, without having to
hardwire a bazillion contingencies into it.  

So really, even thought the original question was something like "why
does area return this tiny number??", your question of the students is
not really applicable to my own concern.  I KNOW the answer to the first
question, and by using either Pauls st_utm function, or my hypothetical
st_distanceUnits function requires that you implicitly understand that
you at least *may* have different projections. 

Robert W. Burgholzer
Surface Water Modeler
Office of Water Supply and Planning
Virginia Department of Environmental Quality
rwburgholzer at deq.virginia.gov
804-698-4405
Open Source Modeling Tools:
http://sourceforge.net/projects/npsource/

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Bruce Rindahl
Sent: Thursday, May 15, 2008 11:31 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Newbie questions: SRIDs, function return
values

I have to also step in here.  PostGIS is a great tool but it can't teach

you basic GIS concepts if people won't take the time to learn them.  To 
work in mapping and GIS you have to have at least a concept of 
coordinate systems and projections.  Otherwise anything you develop will

still cause confusion for those who don't understand GIS but can grab 
big sets of data and put it into PostGIS as Paul states below.

The original question could be phrased as follows:

"I have a big spreadsheet table of the heights of students in my class.

When I use the spreadsheet function =AVERAGE(...) what units is the 
average in?"
or:
"I have a big spreadsheet table of the heights of students in my class.

The values are mixed between cm, inches, and decimal feet.  Why am I not

getting the correct answer for AVERAGE()?"

Bruce

Paul Ramsey wrote:
> The danger of this approach is that it falls apart as people start
> pushing the limits...
>
> select blah from a, b where
st_dwithin(st_utm(a.geom),st_utm(b.goem),100)
>
> It's a pandora's box... as are all things geodetic
>
> p
>
>   

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