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From: Jeff Liebermann <jeffl@cruzio.com>
Newsgroups: rec.bicycles.tech
Subject: Re: Garmin altitude problems
Date: Thu, 08 Aug 2024 08:22:35 -0700
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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.


-- 
Jeff Liebermann                 jeffl@cruzio.com
PO Box 272      http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann      AE6KS    831-336-2558