[ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter

[Derek Atkins]

Ron,

"Ron Frazier (ALE)" <atllinuxenthinfo at techstarship.com> writes:

> I don't think Cumming has an airport, but I'll keep it in mind.
>
> I took the GPS out with me to supper.  When I left, my house was at 1210 feet above sea level.  By the time I got back, it had sunk (on the display) to 1170 feet.  In the last 20 minutes of me sitting here, it has raised back up to 1185 feet.
>
> So, I guess I cannot expect too much from the elevation function of this device.
>
> Sincerely,
>
> Ron

The issue is that the barometric pressure is constantly changing.
Pilots use barometric altimeters, and the rule is that they need to
reset them every 100 miles or every hour.  An airport will report the
current altimeter setting every hour, but some automated systems will
report it every minute, and yes, it can change rapidly if you have
strong storm systems.

Basically, this is a long-winded way of saying that you need to be
constantly resetting altimeters.  When they are set properly they are
very accurate.  However they need to be reset frequently to remain
accurate.

One tip: the barometric pressure doesn't usually change significantly
over short distances.  Most of the local airports will have the same
setting, so you could theoretically just dial the weather at, say,
Kennesaw (770-425-3406), to get a local altimeter setting that should be
"close enough" for you.  Worst case you'll be maybe +/-50 feet off, but
is that really too much error for your use-case?

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available