[postgis-users] SRID for analyzing a USA national data set in Meters

[Pedro Doria Meunier]

Hey Mike,

The only global system, adopted by 1984, is WGS84 (this has been revised in
2004 and it's valid until 2010).
This, as you well know, is a system whose units are expressed in degrees.
If you wish to work in metres you must use a 'localized' system (projected).
The correct geoid must be used for accurate calculations.

So my advice to you is defining 'work areas' (as in regions) if you really
need to work with metres..

This comes from my notes:
"an inaccurate way for calculating the distance would be:
distance [m] = 6378137.0 [m] * Pi * distance [degree] / 180.0
6378137.0 = earth radius

accurate measurement involves the correct geoid to be used (UTM projection)"

INACURRACY ALARM! The above method is horribly inaccurate...

Depending on your task you can programmatically (as in using PHP or whatever
lang) process your points.
Could you elaborate on the task at hand?

Pedro.


-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Michael
Frumin
Sent: segunda-feira, 2 de Abril de 2007 16:13
To: postgis-users at postgis.refractions.net
Subject: Re: [postgis-users] SRID for analyzing a USA national data set in
Meters

pedro,

This appears to suggest that i should pick an SRID based on one point in 
my data set.  However, since the data set is national, it includes 
points from all around the USA (i.e. at least 4 UTM zones).  So, if I 
pick the SRID for, say, Mountain Time, will that horribly distort points 
on the east and/or west coast?  is there no SRID that is perhaps less 
accurate in any one time zone, but more accurate across all time zones 
in the [continental] US?

thanks,
mike