Garmin 800 elevation issues - going mining

bails87

Wondered if anyone could help, I've been noticing a weird elevation issue on my Garmin 800 (6 months old).

I don't often pay attention to the elevation data on the commute, but several times now I've noticed that I've been riding along at what should be around 50m above sea level, but the elevation is showing as anywhere between 10 and 30m below sea level. This is on a normal, straight, flat road with no real tree cover to disrupt the GPS signal.

I also turned the unit on one evening to ride home and after a couple of minutes faffing was about to set off when I saw the elevation change (showing as 80m IIRC, real elevation was probably ~60m). As I stood there, with the unit showing as having full GPS signal, not moving and no speed showing, the elevation fell by a metre every couple of seconds all the way down to about 30m.

Anyone else experienced anything similar? Any ideas on what I can do to fix it. I've tried manually setting the elevation to the correct value but it doesn't seem to stick.

bobbygloss

It sometimes happens on my Edge 500 too. There's a section beside the sea, and the recorded elevation can vary quite widely from -50 to +100m, even though the shape of the profile for the whole ride looks normal.
Also, sometimes the elevation will suddenly ramp up 100m or more, and Strava finds a Cat 4 hill out of nowhere.

Both Strava and Garmin connect have software to correct this, but it makes the profile more jagged.

FJJ

IIRC the Garmin has a barometric altimeter, maybe it is affected by weather (specifically air pressure)? Not sure how much of an effect that would have... Mine seems reasonably accurate although total climb on a ~15 mile journey is usually about 100ft off what RWGPS or Garmin Connect suggest.

meanredspider

Yup - barometric so should be good. There's a calibration step you can do at a point of know altitude. Weather front (with plunging pressure like this morning) can adversly impact it.

The only issue I've noticed with mine is Garmin Connect scales the elevation graph to make my ride look almost flat - Everest & that Altlantic trench thing would fit on there with room to spare.... :roll:

bails87

I don't use Garmin connect. Just upload straight to Strava/Endomondo. Endomondo definitely shows my underground trips, not sure about Strava.

I could understand it varying the height of a known ponit by a few metres depending on the atmospheric pressure, but it was the standing still and watching myself drop by a metre per second or two that was the oddest thing.

...That must have been what it was doing before actually. I start off at 60m, start riding along the flat and the unit drops 40m for no reason, so now I'm at a displayed 20m (real 60m). Then I really descend 30m and all of a sudden I'm being told I'm 10m below sea level.

meanredspider

I get bizarre results on Strava for temperature - often somewhere around 400F! Connect gets it right though I find it takes about 30 minutes for the reading to drop from room temp to ambient outdoor temp