Re: Garmin altitude problems

On 8/10/2024 12:21 PM, Jeff Liebermann wrote:

 GPS is far more accurate measuring 2D surface distances than 3D
measuring altitude.
"Grade, Elevation, and GPS Accuracy FAQ"
<https://support.ridewithgps.com/hc/en-us/articles/4419010957467-Grade-Elevation-and-GPS-Accuracy-FAQ>
 Garmin rates their GPS elevation accuracy at +/-400 ft and their
barometric elevation accuracy at +/-50 ft.
 If anyone wants me to explain why in excruciating detail if anyone is
interested.  The short version is that GPS works by measuring the
Doppler shifts produced by multiple satellites across the sky.  The
motion of the satellite has to be mostly toward or away from the GPS
receiver.  For altitude, that happens best when the satellites are
near the horizon.  A satellite directly overhead does not move toward
or away from the GPS receiver and is therefore useless for computing
altitude.  Since the horizon tends to be cluttered with mountains,
trees, buildings, reflections etc, the signals from the best
positioned satellites can't be used.  Just to make things difficult,
the better GPS antennas are designed to suppress ground reflections
which makes using low elevation satellites even more difficult to use.

IIRC, Jobst explained that with some trigonometric detail many years ago.
And it's all irrelevant for Florida bike paths.
--
- Frank Krygowski