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Garmin Edge dropping grade display

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Garmin Edge dropping grade display

Old 10-13-12, 07:02 PM
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karinbur
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Garmin Edge dropping grade display

I have an older Edge 500. Works great.

I recently received an 800. When I climb steep grades, I lose the grade display. Admittedly, we're often climbing very slowly, sometimes dropping under 3mph, which seems to be the cutoff. But my old Edge 500 doesn't care how slowly I'm going.

A friend bought a new Edge 500. He too is having the problem of the grade display dropping off.

Is there a setting that we're not looking at? Or has the software changed between my older 500 and the newer 800 & 500? We are not using the GSC-10.

Thanks for any input.
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Old 10-14-12, 08:51 AM
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Grade is really hard for a GPS to measure. First, the ability of the GPS to measure - at best - is something like +/-13 feet. So that can translate into a big grade difference. What Garmin does is have a barometric sensor in the GPS to help figure this out but it's also not perfect and is subject to changes in air pressure. It helps but still not great.

So, since grade is expressed as rise over run and as a percentage, that 13' variation can be a problem. If you are in difficult GPS reception territory and that +/-13' turns into something much large (common), then the number is meaningless.

I suspect that's what's behind the problem. The GPS cannot get an accurate enough vertical position fix and they'd rather not report a number than something wildly inaccurate.

J.
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Old 10-14-12, 02:56 PM
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My Edge 705 is perfectly happy with reporting a number that is wildly inaccurate. That said, it has the barometric sensor so it shouldn't be subject to the large inaccuracies that GPSs typically have in the altitude department. Even so, as Johnj80 said, it's a difficult thing for a GPS to determine accurately.

That said, the grade display only works when it's actively recording data points (when recording is on rather than off, an odd restriction, but whatever) and occasionally it stops reporting the grade entirely until the entire thing is shut down and restarted.

I'm sort of surprised that nobody makes a grade display that mounts to your top bar, measures it's orientation with sensors like a smartphone has and is calibrated simply by riding your bike around in a few circles on a relatively flat parking lot. Wouldn't be difficult to make and could easily be quite accurate.
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Old 10-14-12, 07:20 PM
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I'm thinking that the 500 and the 800 have newer software and a more powerful microcontroller. I also have a 705 and it's happy being bizarre. They may be using the newer CPU to make some judgments on the accuracy of the data reported by the sensors and doing that on the fly.

I think the issue with the display you discuss is the sensors that are available. There is a tradeoff between power consumption with some of the accelerometers that are used (require heating to measure the movement of the hot air over a silicon sensor). So you can get a much more accurate sensor but it comes at the cost of higher power consumption.

FWIW, I've started using Cyclemeter on my iphone talking to wahoo fitness bluetooth sensors instead of my garmin 705. The latest release of the app, they've added some intelligence to the grade calculation that made a big difference.

J>
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Old 10-14-12, 07:50 PM
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Originally Posted by JohnJ80 View Post
Grade is really hard for a GPS to measure. First, the ability of the GPS to measure - at best - is something like +/-13 feet. So that can translate into a big grade difference. What Garmin does is have a barometric sensor in the GPS to help figure this out but it's also not perfect and is subject to changes in air pressure. It helps but still not great.

So, since grade is expressed as rise over run and as a percentage, that 13' variation can be a problem. If you are in difficult GPS reception territory and that +/-13' turns into something much large (common), then the number is meaningless.

I suspect that's what's behind the problem. The GPS cannot get an accurate enough vertical position fix and they'd rather not report a number than something wildly inaccurate.

J.
FWIW, the grade readings in Garmin devices come from the built-in barometric altimeter and have nothing to do with GPS. I agree with others that the numbers aren't necessarily that reliable, which is why I always update my Strava uploads to include map-based altitude information rather than the built-in Garmin readings.
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Old 10-14-12, 07:59 PM
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I believe the Garmin readings are a combination of GPS and barometric.

J.
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Old 10-14-12, 08:39 PM
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cafzali
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No, John, honestly they're not. A GPS device like a Garmin uses GPS coordinates to "plot" your current location on a map, which in the case of a Garmin is either the base map or the full overlay maps on a card. That's all a GPS can do, which is why even to this day commercial airliners, which have had GPS and autopilot capabilities in them for years still use Pitot tubes to calculate their elevation.

Consumer-class GPS devices like a Garmin add the elevation function through use of a barometric altimeter. Were it not for that, a Garmin couldn't give you that info. The URL below is talking about a different Garmin product, but references the use of a b.a. to calculate elevation:

https://garmin.blogs.com/my_weblog/page/24/
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Old 10-14-12, 09:18 PM
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My 500 and 800 both cut out grade calc around 3 mph which I notice more walking then cycling. Both have the current firmware but maybe your 500 is an older firmware?
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Old 10-14-12, 09:29 PM
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Originally Posted by cafzali View Post
No, John, honestly they're not.
No, honestly, they must be [a combination of GPS and barometric data].

After all, the grade is your vertical speed divided by your total speed, and the Garmin uses the GPS to determine your total (mostly horizontal, of course) speed.

It does have the option of using it's barometer to determine your vertical speed (and that gives more accurate results in the GPSs that have one), but not your total speed. If it's got a sensor on your wheel it could use that to determine your speed, but considering that it works without that ... it's obviously optional.
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