filtering motorized ride segments with power estimation: finally done (for now)

I implemented the biexponential filter, along with power filtering, in my motorized segments detection code. This program, along with my other Garmin Perl codes, can be found here.

I made a few changes from previous descriptions. One is I changed the anaerobic time constant to 120. This is more the upper end, rather than the median, of typical numbers. This may provide a bit more margin against falsely identifying a segment as motorized.

The other change I made was to separate the altitude-smoothing time constant from the anaerobic time constant. Oversmoothing of altitude can result in the overprediction of power when a rider descends small dips. I set the default to 30 seconds. I also added some speed smoothing but only 5 seconds. Without any speed smoothing and there's too much effect from when the Garmin occasionally spits out a single point with ridiculously high speed. But too much longer than 5 seconds and I lost more of the train segments in Italy where the train was making frequent stops and passing through tunnels:


partial tagging of train travel in Italy

Impressive how the train maintains speed constant to the precision of the Garmin...

With these parameter settings, of all the rides I tested segments which tagged as motorized with a speed threshold of 14 m/sec also did so with a power threshold of 6.5 W/kg, and vice-versa. This isn't too surprising, as I've not taken motorized transport up any significant climbs with my Garmin running (if I take motorized transport on my own, it's almost always a train, and the train well exceeds 14 m/sec). Were I to drive up a winding road like Old La Honda, the power criterion would make a difference. Even a car is unlikely to average 14 m/sec up the road, but on the other hand even a pro cyclist with unrestricted pharmacutical access wouldn't be able to exceed 7.4 meters/second.

Back to altitude filtering: here's some examples of the biexpontial filter I so painfully described. First that trainer ride:


Garmin Edge 500 data from riding a trainer, with smoothing

Here you can see that the 30-second smoothing doesn't do as good of a job as the 90-second smoothing, but still not too bad: the early rapid jumps in altitude result in around a peak artificial grade of 0.5%, which corresponds to an extra 0.5 W/kg which results in a VAM of increase of around 120 meters/hour from these data, or around 0.36 W/kg for less than 30 seconds given reasonable assumptions about bike mass to body mass ratio.

And here's another ride, where I show the result of applying different time constants to the resulting VAM:


VAM from an outdoor ride, with different biexponential time constants.

Of course, the resulting power estimation knows nothing about cross-winds, head-winds, rough versus smooth roads, drafting, or mass changes. But the goal here is to simply identify "extraordinarily high" power. For that purpose I think it does fairly well.

P.S. It would be fairly easy to modify the code to add an estimated "Power" field to a FIT file for rides where you want power but didn't have a power meter. However, I'll pass on this one, as I don't think power estimation from speed & altitude is good enough: it would just result in data contamination.