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Discussion Starter · #1 ·
After losing my Garmin 500 to theft last summer I purchased an 810, figuring the maps and bluetooth would be useful.

I'd never thought much about the barometric altimeter before, but after a few weeks of using the 810, I was out on a group ride we got caught up in hours of drizzle/rain and I noticed that the Gradient reading got whacked - my Garmin was showing say -2% grade when we were climbing +10% hills etc.

Despite my initial worries about water ingress, I dried the 810 out and it continued working ok, but thinking the screen was leaking, I resorted to putting a ziplock bag over the 810 whenever it rained, which hampered its use.

In the last couple of months I've noticed more oddball behavior and also discovered the barometer pinhole on the underside of the unit.

There's an ~80 mile "flat" route that I've completed 4 times since last summer. When I first did the route last June, the Garmin 810 logged ~2,100' of climbing.

My 810 crashed (white-screen) halfway along the route in December, but when I rode the same route on a wet/windy day in January my Garmin logged ~1,450' climbed, and the elevation profile uploaded to Garmin Connect was seriously f'd.

After exchanging email with Garmin support, they told me to set a known location and elevation and thus calibrate the unit.

After following their instructions, my 810 self-calibrates every time I leave the house and displays something to the effect of "calibrating elevation to location X"

We rode the "flat" route a couple of weeks back and my Garmin duly logged ~2,100' of climbing, so I thought everything was good until last weekend.

Our Saturday (dry but windy) group ride was ~90 miles and my 810 logged only ~5,400' which seemed OK, until a buddy confirmed that his Garmin 500 logged just over 6,000' for the EXACT same ride that day.

Questions

1) Is anyone else experiencing elevation issues with an 810?

2) Is this a known issue for these 810 devices?

3) Since my issue is more prominent during wet and/or windy weather (i.e. barometric pressure changes), would a silicone case likely help?

4) After their advice didn't seem to resolve it, should I try to get a replacement unit under warranty?
 

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I have an 810. It has crashed twice in about 10,000 miles. Based on my experience, I would buy another one.

I don't recall my settings. But, do disable auto-shut off.

The elevation is determined by changes in atmospheric pressure. So, changes in atmospheric pressure caused by actual elevation changes and weather impact the units reported elevation. I remember some conversations that the some of the cases/silicone covers things can cover/block the pressure sensor enough to cause weird readings. After uploading to a garmin connect, you can get corrected elevation. In summary, your unit is not broken it is an inherent short-coming of measuring elevation changes with barometric pressure.

Regarding weather - I have had no rain ingress problems. If water is getting in the screen or something, the unit may have a problem. If you really have to ride with a bag over it, it is broken.
 

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Discussion Starter · #4 ·
Yeah, I generally like the 810 and despite some occasional bluetooth connection issues and this elevation matter, it's been fairly stable on v3.62beta firmware. My 810 only crashed once, after which and I took the v5.0 update, which seems to have helped bluetooth somewhat. I also have auto shut-off disabled.

Despite my initial fears, there doesn't appear to be a water ingress problem and since discovering the barometer sensor pinhole, I've stopped covering the unit with a ziplock on wet rides. Concerned about the silicone covers, I've fashioned a small 'cup' from electrical tape to prevent water ingress at the pinhole.

I never questioned grade/elevation with my Garmin 500, so it still bothers me that my 810 misbehaves in the same conditions my 500 was fine, plus a 10% delta between my 810 and a buddy's 500 bugs me.

I also try to remember to flick the elevation correction on Garmin Connect, but feel it would be better as a global, instead of per-activity setting.


I did a ride Saturday from my house. When I got back, my house was 400 feet lower than when I started. You may be right...
Did you live in ocean-front property in CA, by any chance? :p
 

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Here is something you should try, every once in a while set the elevation on the garmin. Then get yourself a ladder and have elevation in realtime display, move the unit upwards in 5 feet increments, observe the reading.
 

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I know someone with an 810 that has elevation issues. She doesn't mind because it lets her cheat but it's incredibly off. Strange thing is that it's not always off in the same direction, but it is very consistent.

There's a very flat route she rides that I log about 300 feet of climbing in where her 810 logs about 2000 feet. When she comes my way and rides in the mountains where I log about 6000 feet in a ride she will only log about 800-1000. So it's sort of backwards on her unit. She tries to avoid the mountains so she doesn't care but there's an obvious problem.

She did call Garmin about it, they did tell her to set her location and elevation and such which she did but it doesn't fix anything. Also doing elevation correction on Strava doesn't seem to do anything to help her either I don't think. She claims that she tries to no avail.
 

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I've always notice that Garmin (I have a 500 and 800) is all over the place on elevation. If you load the data into Strava, use their elevation correction feature, that seems to work.

But sometimes, you just have to say, screw it and enjoy the ride.
 

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yes the way the unit can freak out in the rain you'd thought it was made in california.
I've had some problem with gradients in rain but that's about it, modulo atmospheric pressure which is out of their control
 

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Garmin GPS bike computers tend to have buggy software. But even with a perfectly running GPS device, it's complicated.

The elevation correction after an upload is often inaccurate. It partly depends on the terrain. The online maps use known elevation points on the sides of the mountain or hill, and have to estimate the exact elevation of the road surface. It's not a simple problem.

And the raw data from the GPS recording needs some smoothing, and there's different approaches to that, too.

I'd say that being within 10% is "good enough".
 

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Elevation calculations

I posted a comment about using mapping data to correct GPS recordings in this 2012 thread.

And here's Cullen King, the ridewithgps guy, with some heavy duty math on data smoothing: Using Gaussian and sinc filters on elevation data

And my comment on smoothing raw GPS recordings, with some screen shot examples.

"If I upload the same ride to Strava, ridewithgps, and My Tourbook, I never get the same total elevation gain on the three reports, and often even the average speed for the ride is different."​
 

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Why on earth is a GPS device using barometric pressure altitude readings? It should be doing so by triangulation from the satellites.
 

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Why on earth is a GPS device using barometric pressure altitude readings? It should be doing so by triangulation from the satellites.
Thats a good question, I'm guessing the GPS resolution wouldn't be sufficient for decent altitude data often. Maybe barometer plus GPS would be better.

If I suspect the climbing stats are off much I just have Strava correct my altitude stats.
 

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Discussion Starter · #14 ·
OK thanks guys, so it appears I'm not plagued with a rogue unit. I also wonder why a GPS company wouldn't use the built-in GPS receiver for the altimeter. Surely, adding a barometric pressure sensor adds product cost?

My 810 is calibrated to ground level at my house (looked up on the 'net). Since I don't have a super long ladder, I'll test the unit while walking from the basement to the 2nd floor, just to see what it shows.

It's also unfortunate that Garmin Connect doesn't correct the elevation profile when correction is enabled.

Here's a capture from my Feb 13th ride;

Green Blue Yellow Text Colorfulness

...and here's the garbage captured from my partially wet Jan 2nd ride of the same route;

Green Blue Text Slope White

I don't have a Strava account, but I've created routes on RideWithGPS, and often compare the distance and elevation profile to what may Garmin logs. As of yet, I don't upload anything to that site.
 

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Why on earth is a GPS device using barometric pressure altitude readings? It should be doing so by triangulation from the satellites.
GPS elevation calculations are kind of slow.

My older Garmin 705 has a function to store a location, and there's an optional Average button, that accumulates GPS readings to converge to an accurate elevation for that point. It usually takes 20 to 40 seconds for the elevation number to settle down. That result is usually very accurate, within a few feet of USGS topo maps.

My Galaxy S5 phone is a lot faster, maybe 5 to 10 seconds. But that's still a long time when moving on a bike.

Garmin software is pretty lame. You'd think it would re-average at any short pause, then incrementally adjust the current elevation over a minute or two. But no. The 705 only uses the saved elevation points when the recording is starting.
 
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