Garmin Elevation Tracking Problem

My Garmin 810 has been tracking rides and picking up segments no problem since I picked it up. Yesterday was an especially thick rain day and while the GPS locked in fine, at some point it stopped picking up the elevation points.

It basically stopped at 414m of elevation (the first steep drop to shelf) and then just slowly dropped elevation for the rest of the ride, even though we had an ~80 metre climb in the middle of it. I'm just wondering if this is a normal fail mode in inclement weather.

The second climb started at just before the 6km mark on this graph. You can see the elevation just slowly drops. Oddly enough the entire track still worked fine, just the elevation was messed up.

Shouldn't be weather associated, the signal frequency used by the GPS was chosen to minimize the impact of clouds and rain. Elevation data is generally an order of magnitude worse in accuracy than horiz position due to the angles involved, it could just be that it lost the elevation data due to trees/mountains/buildings in the vicinity of your ride and was never able to properly reconnect.

I think it could be weather related. The Garmin 800/810 relies on the barometric altimeter, not the GPS for altitude data. It does not have temperature compensation so any big swings in temp or weather-related pressure could change your readings.

Not much can be done about the stats showing on the GPS device itself, but you could turn on elevation correction in Garmin Connect (disabled by default for devices with barometric altimeters) for your ride and see if the stats are a bit closer to what you expect.

Garmin says:

How accurate is the GPS elevation reading?

GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, they are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.

From:
https://support.garmin.com/support/searchSupport/case.faces?caseId=%7B66f1b0a0-4cd6-11dc-4733-000000000000%7D

Garmin says:

How accurate is the elevation on my Edge 810, 800 or 510?

The Edge 810, 800 and 510 calculate elevation data using a barometric altimeter. Once the device is properly calibrated, the standard area of accuracy will be +/- 10 feet. To ensure that elevation data is accurate at the beginning of an activity, calibrate the elevation manually.

To calibrate the elevation manually:

1.Touch Setup menu (tools icon) on Main menu
2.Touch System
3.Touch GPS
4.Select Set Elevation
5.Set elevation to known elevation of starting point
6.Select check mark
The elevation will then be calibrated from this manually set elevation.

The barometric altimeter is not temperature compensated. Temperature changes in the measuring device will affect the barometric pressure sensor and altimeter readings.

From:
http://support.garmin.com/support/searchSupport/case.faces?caseId=%7Bdf7142e0-ea50-11e0-73d0-000000000000%7D

Thanks for the thoughts here guys. The unit had been performing as expected for the past few weeks, though I did have to manually set the elevation as per the instructions in Craig's second post.

This problem cropped up half way through a ride; before that it was doing just fine. Possible that the barometer got filled with water mid-ride and stopped sensing pressure appropriately?

karma trying to tell you not to care so much? ;)

You're so good at pushing my buttons – could you help me with pushing the Garmin buttons as well?

You're so good at pushing my buttons – could you help me with pushing the Garmin buttons as well?

jabjabjab, I'm all thumbs

This problem cropped up half way through a ride; before that it was doing just fine. Possible that the barometer got filled with water mid-ride and stopped sensing pressure appropriately?

Could be, any idea where the sensor is on the GPS, just wondering if you could protect it somehow?

Depending on when you were riding yesterday, the Barometric pressure was changing during the day:
http://www.worldweatheronline.com/v2/historical-weather.aspx?q=North%20Vancouver,%20Canada

I'm not sure how sensitive the elevation is with this amount of pressure change, anyone know?

According to that, the pressure went from 1015mb at 2pm to 1012mb at 5pm. I was riding from 2 until 3:30. I don't think that's a significant enough change to give me such wonky readings, though the weather was indeed inclement and changes could have been more significant in the area I was riding. That said, I'm more inclined to think this was a moisture-related hardware problem and not actually the barometric pressure doing something wacky.

Threadjack…how do you find the accuracy of the 810 in the woods? I have had a few different 500's and they all suffer from the occasional case of drift where i will totally miss a segment.

Winter forests, rain and cloud all seem to play havoc with my garmins. All have barometiric elevation adjustment, should just smooth out the data instead of big spikes.

Check the little microphone type opening on the back side of the garmin…

According to that, the pressure went from 1015mb at 2pm to 1012mb at 5pm. I was riding from 2 until 3:30. I don't think that's a significant enough change to give me such wonky readings, though the weather was indeed inclement and changes could have been more significant in the area I was riding. That said, I'm more inclined to think this was a moisture-related hardware problem and not actually the barometric pressure doing something wacky.

Morgman, same thing has been happening to me and it has only been since the weather is really crap. Will try what Craig as said but my thoughts is that it is weather related.