What Total Ascent to use?

During cycling, I am using Polar 720 HRM and Garmin Edge 305 GPS, after
the ride, I download to Motion Base site and Training Center Garmin
software, each gives me a different number. Since the difference is up
to 100%, and in my club we are posting this number for each ride...I
need to know the truth...

Today's ride produced the following numbers for total ascent:

Polar: 3265 ft
Edge 305: 4680 ft
Training Center: 4674 ft
Motion Base: 6200 ft

which number is the current number?

Thanks.

Both off the top use a barometer to measure ascent. So, that needs to be corrected. The only way to correct it is to know where you are and look it up on a map, or get the software to do it for you, if it can. So if you use the motionbased gravity web service then the Garmin 305 should be the accurate one.

Is a barometer not accurate?

Not particularly. My elevation is fluttering between -41m and -42m currently.

LOL, riding near the Dead Sea?

__________________
De Rosa Planet
Campagnolo Per Sempre!
PAOLO BETTINI CAMPIONE DEL MONDO x 2!

Well, barometers are not particularly brilliant at absolute altitude readings, but are a reasonably good way to measure relative changes in altitude. The Edge 305 does try to reset itself to the GPS satellites, but altitude is not one of the things GPS is all that good at, so if the unit is moving the algorithms can't quite manage it. Now the Edge 305 keeps a tally of rises and drops from the barometer, but doesn't do it too well, often under reporting rises. Finally I tend to only have the unit on when it is moving. So over time the unit tends to read the elevation as slowly dropping.

Leaving it switched on on top of the car roof with a good view of the sky for an hour or so seems to get it all back into sync.. So I guess it's about time I did that again.

My Polar 720i gives highly reproducible, and believable, total ascents. As another poster has written, they are not so good at absolute altitude, as they vary +/-50-100m with atmospheric pressure. The Polar altitude can be "zeroed" at any site of known altitude, such as your home, before your ride.

__________________
"All that we see and seem is but a dream, within a dream..."

Yes, the Polar is OK for shorter rides, but if you go out all day, do audax, that kind of thing, then you may need to get the data collected corrected, which motionbased can do with the Edge 305. Personally I've given up on the Polar and just use it on my spinning bike as its total data capacity is just not large enough at 4 hours. Yeah, you can shift it to sample at 60secs but that's still not enough for the really long rides I sometimes do, and that sample frequency is not 'fine grained' enough for me..

My 720i has a hell of a lot more capacity than 4 hours, and I'm definitely not sampling at 1/60 Hz, but rather whatever the default is. I can put several full day rides on it before needing to download.

__________________
"All that we see and seem is but a dream, within a dream..."

Depends also on the extras, I have the power meter/cadence thing on as well.