Originally Posted by
unterhausen
I had really weird altitude readings the other day in a thunderstorm. The position tracks were ok, just the altitude was off.
The Garmins use barometric pressure to improve the altitude measurements.
GPS is mostly focused on horizontal positioning (GPS alone is considered poor for altitude measurements).
Also people can tolerate much larger delta errors in positioning than in altitude measurements. Put another way, 5000 feet of altitude is a much bigger deal than 5000 feet of horizontal movement is.
Originally Posted by
eja_ bottecchia
The change in barometric pressure may well explain the higher readings.
The barometric pressure also changes with temperature and humidity. The Garmin 800 have a thermometer, which means it can adjust for temperature. Beyond that, the assumption (made by the device) is that the only parameter affecting barometric pressure is elevation/altitude.
Wacky/rapid changes in the weather might significantly effect the estimation of elevation. I would guess that that would have a larger impact on long climbs than it would on rolling terrain (because the gain/loss happens faster than weather-related barometric changes).