Re: Garmin altitude problems

On Thu, 08 Aug 2024 08:22:35 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Thu, 08 Aug 2024 10:18:39 GMT, Roger Merriman <roger@sarlet.com>
wrote:
>

Jeff Liebermann <jeffl@cruzio.com> wrote:

On Wed, 07 Aug 2024 17:57:59 GMT, Tom Kunich <cyclintom@yahoo.com>
wrote:
 

The formula for change in altitude with air pressure in the lower
atmosphere (about to 35,000 feet) is:

 
Nope.  From what's left of your formula, I'll assume you're trying to
calculate the air pressure at a given altitude.  There is no *change*
in altitude involved.  Looks like you lost all the Greek letter
symbols and formula symbols. 
 

P=Pb[1
Where:
 
 
Pb
TM,b
LM,b
h
hb
R
g0
M

 
That unreadable mess should look something like these:
 
"Atmospheric Pressure Calculator"
<https://calculator.academy/atmospheric-pressure-calculator/>
 
"Air Pressure at Altitude Calculator"
<https://www.omnicalculator.com/physics/air-pressure-at-altitude>
 
Note that you need to know the barometric base pressure (Pb) at sea
level for such a calculation to work.  It's NOT going to work if your
riding up or down a hill where the barometric pressure might change.

 

In fairness the 830 recalculates on start up every so often particularly if
it’s a changed location or it may well do so every time but don’t watch it
start up in general.
>
Roger Merriman

>
Garmin works in mysterious ways.  Garmin's self calibration algorithm
is a bit stranger than one might expect.  For example:
<https://forums.garmin.com/sports-fitness/cycling/f/edge-830/174850/elevation-auto-calibration>
"Every time I start to record an activity on my Edge 830 I get this
message "Elevation calibrated to location" and immediately the
elevation number goes up by +/-12m out of the blue with me standing
still in the same spot.
The problem is that the elevation shown right before I press the
record button (before the "auto-calibration) is more accurate than the
one modified."
"The value is coming from the saved location that is near your
starting point."
>
I seem to recall reading somewhere that Garmin recommends calibration
before starting on a ride.  I'm too lazy to find where Garmin said
that.
>
However, that message is over 5 years old and the problem might have
been fixed long ago with a firmware update.
>
The barometric altimeter accuracy in the Garmin Edge 830 is specified
at +/-50 ft accuracy (which is better than the +/-400 ft accuracy for
the GPS).  There's also a +/-10 ft error for reasons unknown:
<https://support.garmin.com/en-US/?faq=sFMkjQFdnZ99DcunfAue66>
"Elevation accuracy of +/-10 meters is for any given reading during an
activity, not the total elevation gain/loss at the end of an
activity."
>
Very mysterious methinks.

Just how do these garmin gizmos work? Back when I worked on airplanes
the Altimeter measured the altitude above the home field. During
preflight, before engine start, the altimeter was set to "zero" and
during that flight measured height above the location where it was
set.

I believe in more modern times one calls the "Tower" and they give you
the altitude above sea level corrected by current temperature.

Where does the Garmin gets it base altitude from or is "0" simply the
altitude, and temperature,  when/where you turn it on?


 
--
Cheers,

John B.