[talk-au] Overland Track added

John Henderson snowgum at gmx.com
Tue Feb 23 03:49:37 GMT 2010

Steve Bennett wrote:

> Garmin Oregon 550.

I see that's got a barometric altimeter too.  Very good.

> Ah. What is "GPS-derived altitude" though, exactly - does it rely on a
> model of the earth's surface, or is it effectively computing the
> distance from the satellites?

Purely from satellites, definitely.  Others on the list are likely much 
more up to speed with the technicalities.

> Anyway, my recorded elevation for Mt Ossa is 1613m, and the Acropolis
> is 1477m. Wikipedia gives Mt Ossa as 1614, and the Acropolis as 1481m.
> Maybe this method is more accurate than I was expecting...
> OTOH, I'm still not convinced that all this is necessary. Surely using
> figures from an authoritative source would serve everyone a lot
> better...

Yes, the figure from a proper survey should have an accuracy of a few 
centimetres.  One of my many jobs (back in the 70s) was a chainman 
(surveyor's assistant).  With modern electronic gear, I believe there's 
no such job any more.

John H

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