On Mon, Aug 04, 2003 at 11:10:36PM -0500, Dave Huang wrote:
> On Mon, Aug 04, 2003 at 07:25:09PM -0500, Richard Rauch wrote:
> > Anyway, I thought that when the terms "longitude" and "lattitude" were
> > used, one was always refering to the "traditional" system (someplace
> > in Greenwich at longitude 0, the equator at lattitude 0; I don't know
> > where they place the center of the earth for defining angles phi & theta).
> > Sorry for assuming that that was clear.
> 
> AFAIK, Greenwich is longitude 0 and the equator is latitude 0 in all
> systems; the difference between the various systems is more subtle

Okay, perhaps I was conflating two issues that were only vaguely related.
Off-list, Gilbert was giving me some explanation of these issues, and
discussing both UTM/GPS "zones" and this idea of a "datum".

I thought he meant that a "datum" was the reference point for a "zone",
where offsets are measured in meters north/south and east/west.  So when he
insisted that longitude/latitude was useless without knowing the "datum",
I thought that he meant this reference point...


> than that. The earth isn't a sphere; it's obviously quite irregular :)
> However, an ellipsoid is a pretty decent approximation... the
> differences lie in exactly which ellipsoid to pick.
> 
> In any case, as others have mentioned, while the differences are
> important to some people, it doesn't matter for this application... I

Agreed.


> don't think it'll make a pixel of difference on any maps, and if some
> stalker's gonna drive to your place, they won't care if they're off by
> a couple of meters :)

...and if someone wants to aim a missile at me for a "surgical strike",
I won't mind if they miss...  (^&

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/