Re: SRID for analyzing a USA national data set in Meters

> 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@...
> [mailto:postgis-users-bounces@...] On Behalf Of Michael
> Frumin
> Sent: segunda-feira, 2 de Abril de 2007 16:13
> To: postgis-users@...
> 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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