Elevation change errors and their effect on power

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rruff
Posts: 445
Joined: Wed Apr 23, 2008 10:48 am

Elevation change errors and their effect on power

Post by rruff »

Quite awhile back I mentioned that I was seeing up to a 5% scatter in the elevation change on a long climb that I do fairly often. Since the iBike uses barometric elevation readings to determine slope, this can result in a pretty big power error on long climbs (or descents). I thought I'd revisit this to see if there was any way to improve the results.

The clearest website I found regarding barometric altimeter measurements is here: http://www.hills-database.co.uk/altim.html

If I understand correctly, there are significant sources of error even if the barometric sensor is working perfectly. Temperature looks like the most important variable, with humidity also having an effect. The drift in barometric pressure is another potential factor on long rides, but since this climb takes me ~42 minutes, the drift should be quite small.

I tallied some of my trips up this climb below. I also looked to see if there was any correlation between the measured elevation change and two other variables: average iBike temperature during the climb, and the iBike barometric reading. Surprisingly there was a correlation with both... meaning (I think) that there is a chance the iBike could be made more accurate in this respect, using the existing sensors.

Anyone else interested in posting their numbers? I wonder if this phenomena is fairly consistent with different units and different elevations. It should be a climb that is fairly long (10 minutes minimum), that you have ridden numerous times, and can identify the "bottom" and "top" accurately.
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Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 8:43 am

Re: Elevation change errors and their effect on power

Post by Velocomp »

Just so we don't all get wildly confused on this thread, I believe it is a correct statement that this data is from a Gen III iBike.

The Newton uses a completely different baro sensor, so its results may be different.
John Hamann
rruff
Posts: 445
Joined: Wed Apr 23, 2008 10:48 am

Re: Elevation change errors and their effect on power

Post by rruff »

That's true, but I'm guessing it isn't a barometric pressure sensor issue, but rather the calculation process (algorithm). Newton data would be great, but I doubt anyone has had time to do a bunch of long climbs with one yet.
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