Re: Garmin altitude problems

On Fri, 09 Aug 2024 10:44:09 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Fri, 09 Aug 2024 08:14:59 +0700, John B. <slocombjb@gmail.com>
wrote:
>

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?

>
A barometric altimeter works in a similar, but not identical way.  For
Garmin, you calibrate the altimeter from a known altitude location
from a designated reference.  If your starting location is known to be
100 ft AMSL (above mean sea level), you set your Garmin for 100ft and
you're done.  If you don't have an accurate map, using the GPS
altitude, averaged over a long period will suffice, but you need to
enter the datum used to define zero altitude used by the GPS.  Notice
that you don't enter the barometric pressure to make this work.  For
the Garmin 830, in manual mode, it's simply set and ride:
<https://www8.garmin.com/manuals/webhelp/edge830/EN-US/GUID-1833C7CC-667E-48C7-B7D2-7A6041470478.html>
>
On later devices, Garmin uses the GPS altitude and DEM (digital
elevation model) to calibrate the barometer.  This is for the Garmin
Forerunner 965 watch:
<https://www8.garmin.com/manuals/webhelp/GUID-0221611A-992D-495E-8DED-1DD448F7A066/EN-US/GUID-BC734846-01A7-4F33-86D4-DFBDBC06CDB4.html>
Again, notice that barometric pressure is not involved in establishing
a reference altitude.
>
Garmin also has an "auto cal" mode, which does all this automagically.
I couldn't find an explanation on how it works from Garmin.  My
guess(tm) is that it uses the nearest GPS reference available where
the bicycle was not moving and the DOP (dilution of precision) is
sufficiently low for the GPS altitude to be considered accurate.
<https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)>
Again, notice that barometric pressure is not involved in establishing
a reference altitude.
>
Once a reference altitude is established, the barometric pressure
becomes involved.  The math is easy enough:
"Atmospheric Pressure Calculator"
<https://calculator.academy/atmospheric-pressure-calculator/>
"Air Pressure at Altitude Calculator"
<https://www.omnicalculator.com/physics/air-pressure-at-altitude>
For faster response, Garmin might use a lookup table to convert air
pressure to altitude.
>
However, barometric pressure change with weather, location and
environment.  The MEMS pressure sensor might also drift.  Therefore,
the altitude must be recalibrated occasionally.  My guess(tm) is that
Garmin uses the nearest known GPS location, with the lowest DOP, to
establish a new reference.  If the new reference altitude is not quite
perfect, the rider sees a "glitch" or discontinuity in the elevation
profile graph when the reference is reset.  Garmin probably tries to
"smooth" this transition, making the graph look more reasonable.
>
I was wondering why Garmin elevation accuracy specification for the
Edge 830 was +/-400 ft.  GPS elevation accuracy should be about +/-120
ft for most GPS's.  Some possible causes are:
1.  The EDGE 830 was turned off at night and not given sufficient time
to download a few days worth of ephemeris data.  Garmin advised to put
the GPS somewhere with a good view of the sky and let it download for
about 15(?) mins.
2.  The GPS antenna on the 830 is probably tiny.  That will reduce
signal strength and therefore accuracy.
3.  The Sony GPS receiver on the 830 uses GPS and *EITHER* Glonass or
Galileo.  It really should be using all three to see more satellites.

The basic system sounds much like my old airplane altimeter. But how
is atmospheric pressure due to temperature handled?
--
Cheers,

John B.