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

[Paul Ramsey]

Pedro Doria Meunier wrote:
> 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

Use distance_sphere() to get this as a built-in postgis function. 
Distance_spheroid() returns an accurate distance, but at 10 times the 
CPU cost of the sphere calc.

P

> 
> 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
> 
> 
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-- 

   Paul Ramsey
   Refractions Research
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   pramsey at refractions.net
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