CraigB

I have a Garmin 500 that I've been using since early this season. Lately, like on the last couple of rides, the elevation data it compiles has shown an odd little anomaly that I wonder if anyone else has experienced. On both of these rides, I stopped at local cafes, turned off the timer (but let the unit power itself down) and grabbed a quick bite to eat, staying there for maybe 20 minutes or so. WHen done, I get back on the bike, power up the unit, restart the timer and continue on my way. Once home, I upload the data to the Garmin site, and to Strava, and both shown an identical jump in elevation that bears no resemblance to the actual terrain - like a 300' cliff face on the graph. On close inspection, I find these have happened at the point in my ride where I stop to eat and the computer is inactive.

Anyone else have this happening? Any ideas? I've had this happen with the temperature data because the unit has been sitting in the sun, but never with elevation. In neither case was there a change in weather while I was stopped, so I don't think any atmospheric pressure differences are accounting for it. I never really relied too much on the elevation data to begin with, but with Hilly Hundred coming up in a few weeks, I'd like to think it would give me some more realistic data.

CraigB

FYI, here are the elevation graphs for yesterday's ride:

Garmin's "corrections enabled" graph:


Garmin's "corrections disabled" graph:


And Strava's graph:


Interestingly, I just noticed that the "blip" in the data shows up in slightly different places on the two Garmin graphs.

volosong

That sux. Only thing I can suggest is to save the file as a GPX and then open it into an editor to look at the data. There are a bunch of freeware GPX editors out there.

If your bike is in a safe location, just leave your computer on, but make sure "autopause" is enabled. I'm pretty sure it won't power down in that mode, but would have to check the manual to make sure.

Can you send your FIT file to Garmin and ask them what's up?

gregf83 

That's normal for any barometric based altimeter. If it's a problem you can program in the actual elevation at the coffee shop you stop at and the 500 will correct itself.

edit: sorry I originally just looked a the blip and didn't notice that it was 300'. If it is always that much after a 20 min stop I would talk to Garmin. Perhaps you have a defective unit.

Barrettscv 

My 800 also has strange elevation profiles. For me, my starting and ending elevation never match and it's the same location.

Garmin uses both GPS and a barometric sensor to determine elevation. I assume that the software or the sensor is less than perfect.

BikeWNC

Maybe you are getting some interference causing the anomaly. Or as Greg said, the unit has to reestablish and stabilize when it comes back on. Try leaving the unit on while doing the ride and see if the same thing happens.

stapfam

New to a 500 aswell and I noticed that on one ride total elevation recorded on a ride was 109ft. Yet on the ride there was one climb that had 140 ft elevation to it. That is besides all the other small climbs that were done on the ride aswell. And it was a non-stop ride starting and finishing at the same point.

RedC

Most of my rides include a breakfast stop ( I ride 3 or 4 days a week with a group that calls itself "The Breakfast Club") and I never shut my garmin off nor does anyone else in my group

MinnMan

Why shut it off mid-ride? The power savings are negligible and then it has to relocate satellites when you turn it back on.

The difference you see is surprisingly large, but probably is owing in part to the pre-stop readings coming from the barometer and the post-stop readings beginning at whatever elevation it gets from its new satellite read.

woodway

Garmin's elevation correction just ignores the altitude data from the device and calculates altitude using a US topo database that has data points every ten meters or so.

The barometric altimeter needs time to calibrate when you first turn the device on. This can take some time. So if you want to turn the device on and go, but still get accurate elevation you need to manually calibrate the altimeter.

Here is the Garmin FAQ on the topic:

CraigB

Maybe I wasn't clear in my OP. I don't shut the unit off. I turn off the timer or stopwatch function or whatever you call it. But when I stop like this, I'm off the bike long enough for the unit, sensing no activity, to shut itself off. If it's possible to change the default amount of time before that happens, I'll give it a try. But no, I don't deliberately turn the unit off myself.

volosong

Your OP was clear enough to me, Craig. Take a test ride and let the unit "autopause", (i.e., don't stop the timer).

AzTallRider

If you don't stop the timer, but instead use auto-pause, it won't shut itself off.

AzTallRider

And by the way, all of the methods of calculating altitude, and gain during a ride, have their issues. Barometric pressure and GPS fluctuate while you stay in one place, and topo info can get confused by such things as bridges crassing a ravine. As an engineer in my club who gave a longer explanation said: "pick the one that gives you best number for the day, and leave it at that!"

CraigB

OK, I've turned "Auto Power Down" off, so it won't shut itself off due to inactivity, and I've enabled auto-pause. We'll see if that works on the next ride that's long enough to include a stop.

volosong

Please let us know. You don't need to worry about battery power. They are good for something like 18 hours, right? When I return from a ride, one of the first things I do after changing and showering is to upload my ride to either Strava or Garmin Connect, (usually Strava). Then, I put it on the a/c charger so it will be ready for the next ride.

Last Friday, after a self-supported century, I was too pooped and didn't have time to put my Garmin on the charger, (the rear and front strobes came first - safety is more important to me). When I needed to use it the following day, there was still plenty of battery power for a several hour ride. Unless you're doing a multi-day tour, I doubt you'll ever deplete the battery, (assuming you re-charge after every ride).

billydonn

I'm betting that solves your problem. Let us know.

rm -rf

Was this ride near Indianapolis? The actual elevations there are near 800 feet. It looks like it was low at the start, and corrected during the stop.

My 705 doesn't do corrections once it's started. The elevations around here are 500-600 feet, but sometimes it will show low 300 feet elevations when I turn it on.

A 705 has a Save Location method: Menu-->Save Location -->Average. It starts counting GPS readings and adjusting the calculated elevation. If I let it sit for 60 seconds or so, the elevation reading converges to an accurate number. I hit OK to save it.

Then, if I start a ride within 100 feet of a saved location, it instantly changes the elevation to match the saved location. It won't use any saved locations that I ride past while it's running, only one in range right at the start.

It doesn't look like the 500 has this averaging elevation method, it just has saving a location and entering an elevation number by hand.

Perhaps your 500 will adjust it's elevation if you turn it on and let it sit for 10 or 15 minutes before hitting start. You could test it at home. That's how a GPS should work.

I never seem to be organized enough to turn on my 705 that far in advance, but the few times I've left it for 5 or 10 minutes, it didn't seem to adjust all that much.

rcb78

Something no one has mentioned yet is that the altimeters in the Garmins are not temperature compensated. So if there is a temp change when the unit is not recording, the elevation 'reading' will change while just sitting there. They also do funny things when the weather changes while you're out thanks to the pressure change. The micro climates in the Bay Area really highlight this.

CraigB

The ride was from the north edge of Indy proper up to a small town about 20 miles north, and back home again.

I understand that the data can be affected by changes in barometric pressure, like when a front rolls through, but in both cases where this has happened there has been no change in the weather. I didn't even think, though, that temperature changes might affect it. The first time it happened, I was stopped for breakfast and the ambient temps might have increased 6-8 degrees F during the stop as the morning warmed up. This last time (the ride from which the graphs were taken), there was no change in the actual air temperature. Having said that, however, on both rides the unit was sitting in sun during the stops, and the temperature graphs show an instant increase in temperature while at the stop, followed by a gradual decline once I started riding again. So at least the temperature readings seem to be affected by sitting in the sun.

qcpmsame 

This is what I was thinking about. Did you leave the bike/unit outside or take it inside while you were at the cafe? An abrupt temp change by going into an air conditioned building from the outside might confuse the barometer since it isn't temp compensated. Just a thought.

Bill

rcb78

You've got your answer then. The Garmin doesn't really read ambient temp, it reads heat soak. Move it from shade to sun, give it a few minutes to normalize and you'll see this. It makes rides look MUCH hotter than they are if you spend significant time in direct sunlight. Anyways, what you're describing sounds completely normal given the way the Garmin was designed.

dck

All GPS devices require at least 3 satellite fixes to calculate location, and 4 satellite fixes to calculate elevation. Accuracy of the calculation depends on a lot of variables, but neither the position and elevation are completely accurate. After you shut it off, it takes a while to re-sync with the satellites, and even then it may produce a slightly different location or elevation.

CraigB

Sorry for the delay, everyone, but this morning's ride was the first one in the past week that was long enough for a rest (read: breakfast) stop. Turning off "auto power down" and turning on "auto pause" seemed to do the trick. No spurious elevation readings at the stop. I left the Garmin on the bike, parked in the sun as usual, and all the report showed was the expected bump in temperature.

Here's the correction-enabled Garmin elevations graph:




And here's the same from Strava:

volosong

Thanks for the feedback. I wish all issues were as easy to solve. Ride safe.