Motion Based Question

[Veronica]

How "off" is their altitude data? I know I've seen references to MB over reporting climb.

Looking at some of Jo's comparisons in her journal between her Ciclosport and her Garmin, it looks like Garmin can be off by as much as 45%. That's a huge difference!

Veronica

[maillotpois]

It's not really a Garmin issue, because the barometric adjusted Garmin data run through Garmin training center is pretty spot on - consistent with my old HAC4 data for the same rides and with other people's Polars, etc.

The problem is putting the data into MB without checking the new adjust feature in MB (I don't know what the new button is but there is now some option to allow MB to track more closely with the barometric data somehow. I don't use it so I don't know what it's called. We have a teammate who would always log our training rides and then broadcast his MB data and it would be at least 15% higher - really depended on the ride but it was always WAY higher. When he checked the adjust button on MB it went a lot closer to the GTC damat, but still maybe 5% high.)

[jobob]

I'm seeing that discrepancy between the elevation data from Garmin Edge 305 and my Ciclosport when I only run the Garmin data thru Training Center.

Through MotionBased, bah, forget about it! (unless you want to impress yourself with how much you really don't climb ) I didn't know there was a way to correct it though - I'll look for that adjust button.

The data on my Ciclosport tends to agree moderately well with Bikely mapping, and if anything might be a bit on the low side (maybe 10% lower than elevation gains from Polar). So as long as it still works, I'll take elevation gain off my Ciclosport.

[OakLeaf]

I just went looking for it again. Up at the top right of your Activity Dashboard there's an icon called "Activity Options." Click that. Then for a Garmin Edge with barometric altimeter, check "Auto-Correct GPS Signal Errors" and uncheck "Use MB Gravity Elevation Correction."

I can't attest to the accuracy of that vs. SportTracks correction vs. Training Center, though.

[jobob]

Cool, thanks, I'll give it a try.

[SadieKate]

Based on the statistical average of 1 ride (today's drop run in which I cleaned rock gardens I never had*):

SW/Ascent/Descent

SportTracks / 1,569 / 4,118 (using data smoothing)
GTC / 1,634 / 4,182
MB with both autocorrection and MB gravity / 2,029 / 4,614
MB with auto correction / 2,195 / 4,747
MB with all elevation corrections disabled / 2,263 / 4,815 (check mark cleared in auto correction and grayed out in MB gravity so that the page shows "corrections disabled)

I think I'll stick with GTC and SportTracks (for my Edge 305).

*just had to get that in there!

[SadieKate]

Also, GTC and SportTracks report a max speed of 21.5 and 21.1. respectively. MB reports 27.2. Bwahahahaha!

[OakLeaf]

MB specifically says to DISable the Gravity correction when you're using Edge 305/705 (with barometric altimeter). What happens when you enable the autocorrection but disable Gravity (looks like the one thing you didn't try)?

Also, what level of smoothing do you use in SportTracks? I have mine off for elevation, off for cadence, on at 3 (of 150) for speed, on at 10 for HR. I think I'm going to increase the HR smoothing, but it seems to work pretty well for speed - just eliminates the obvious spikes and not the momentary fast descents.

SportTracks worked so well to correct distance when I was having that software bug, that I'm inclined to trust its elevation data as well (with smoothing off, especially important in my terrain of short, steep hills).

[SadieKate]

MB specifically says to DISable the Gravity correction when you're using Edge 305/705 (with barometric altimeter). What happens when you enable the autocorrection but disable Gravity (looks like the one thing you didn't try)?

Yes, I did. It was just mis-typed list. See above. I verified again.

[SadieKate]

Here's a ride jobob can use for comparisons and demonstrates the erratic nature of MB:

Crater Lake Century
SportTracks / 6,222 / 6,230 (using data smoothing)
GTC / 6,218 / 6,225
MB with both autocorrection and MB gravity / 8,053 / 8,053
MB with auto correction / 6,490 / 6,497
MB with all elevation corrections disabled / 6,567 / 6,573 (check mark cleared in auto correction and grayed out in MB gravity so that the page shows "corrections disabled)

[Mr. Bloom]

I just went looking for it again. Up at the top right of your Activity Dashboard there's an icon called "Activity Options." Click that. Then for a Garmin Edge with barometric altimeter, check "Auto-Correct GPS Signal Errors" and uncheck "Use MB Gravity Elevation Correction."

I can't attest to the accuracy of that vs. SportTracks correction vs. Training Center, though.

I made the change too and this brought the climbing down to what I had last year on my Hilly Hundred route.

But, this is still 15% higher than the GeoBike Profile provided by the club.

HOWEVER, when I compare the MB elevation profile to the GeoBike Profile, the peaks and valleys track with precise accuracy.

So, this seems to indicate to me that the elevation profile is accurate, but the cumulative count is off in some way.

Does this make sense?

[OakLeaf]

How did the club obtain their data?

[TrekTheKaty]

I have a Garmin Forerunner 305. I've never paid much attention to the altitude until my century. I got a 6800 ft altitude change, and saw others as low as 5800 ft altitude change on the same ride. I have the autocorrect on because it is recommended. When I turned on elevation correction, it lowered it to 6300 ft.

I'm also curious what other do about this. How do I know what's accurate?

[SadieKate]

Mr. Silver, your results absolutely make sense. It's all on the algorithms, and how frequently the device is recording and a number of other whizbang mathematical stuff that confuses me. You'll see the same problem when comparing to many websites that are just using GPS data. So read onward . . . .

cunninghamair, barometric altimeters must be used to get readings at all accurate. The Forerunner 305 does not have one. Do some searches on this forum and over on bikejournal for more discussion.

I've used Polar and Ciclo which also have barometric altimeters with very consistent results with my Edge 305 run through GTC and now SportTracks. So, I'm comfortable using those numbers. Accuracy is in the eye of the beholder, it's pretty unanimous that a barometric altimeter is required to get close. Just remember that weather patterns can cause some inconsistency, but generally not as much as GPS-only data.

[OakLeaf]

The Forerunners do not have the barometric altimeter, so for those, MotionBased recommends that you turn on the Gravity correction.

We went for a hike in the woods this afternoon. I was hoping the slower speeds of hiking might bring all the results closer - not so. I brought my Edge 705 with me (partly for the barometric altimeter, partly because of the receiver that's much more sensitive than my Forerunner 301). Even so, with the tree cover, the reception was poor. So everything's going to be off - but FWIW, the data displayed in the unit called it 5.1 miles with 727 feet of elevation gain. GTC calls it 5.1 miles with 863' of climbing (and 859' of descent to the start point, so pretty close on the net anyhow). SportTracks calls it 5.98 miles with 1053' climbing (but only 956' descent!). MB, with autocorrect on and Gravity off, calls it 5.94 miles and +1076/-1050.

As far as speed smoothing, GTC had a max speed of 4.5 mph (probably about right); ST says max speed of 19.1 mph! (I hate to turn the smoothing up because it honestly seems pretty accurate on the bike - wish it would let you set smoothing differently for different sports). MB didn't smooth that spike out too well either - it gives me a max speed of 17.2.

What's the most accurate? Who knows - only thing I know for sure is that the max speeds on ST and MB are in need of some serious smoothing.

EDIT: I realized that I still had my 705 turned to smart recording. Probably for a hike, especially under tree cover, it would've been a lot better to set it for 1 second recording. Doh.