Two questions:
First, I've noticed that my elevation values always show I am about 40-50 m below sea level. They should be 30-65 m above. Is there any way to fix this discrepancy? I understand that absolute elevation does not affect power ratings and values are in part a function of changes in weather conditions, but I don't understand why that always gives me negative values.
Secondly, starting and end elevation points are often out from each other by 5-10 m in an out and back ride. When I use this to tweak my calibration, this means it keeps adjusting the tilt value. This does affect power values, usually around 5-10 W difference. What gives?
Third bonus question - does it make sense to keep tweaking your calibration ride after ride, or should I just do one or two tweaks and call it a day? What should I be looking for in these tweaks to indicate it worked well?
Elevation values always negative
Re: Elevation values always negative
1) Elevation readings are based on baro sensor readings. Baro readings vary with weather.
You can adjust your absolute elevation, after each ride, with this command:
In this ride file I changed starting elevation from -128 feet to +10 feet
Note that changing the absolute elevation won't affect power readings
2) When you "tweak" the ride with Check Calibration or Analyze Route, you're also tweaking wind speed readings. This is likely the thing affecting power values.
3) Once you get your device dialed in, I would leave the tweaking alone. To compare results across rides, you need your settings to be the same.
The thing I look for is wind speed. On out-and-back rides, average wind speed should be close to average bike speed. NOTE: wind conditions can change during long rides, so this is an approximate, not exact, rule.
You can adjust your absolute elevation, after each ride, with this command:
- Screen Shot 2020-04-19 at 3.20.06 PM.png (106.32 KiB) Viewed 2827 times
- Screen Shot 2020-04-19 at 3.20.34 PM.png (142.35 KiB) Viewed 2827 times
2) When you "tweak" the ride with Check Calibration or Analyze Route, you're also tweaking wind speed readings. This is likely the thing affecting power values.
3) Once you get your device dialed in, I would leave the tweaking alone. To compare results across rides, you need your settings to be the same.
The thing I look for is wind speed. On out-and-back rides, average wind speed should be close to average bike speed. NOTE: wind conditions can change during long rides, so this is an approximate, not exact, rule.
John Hamann
Re: Elevation values always negative
Great, thanks John. I'll keep the tweaking to a minimum. I should have mentioned I've adjusted the elevation using the method you show but was wondering if there was a more "permanent" fix. I suppose if pressure changes regularly with weather then that's not meaningfully possible, but it must default to assuming a specific pressure is sea-level e.g. 101.3 kPa.
>Once you get your device dialed in, I would leave the tweaking alone.
So do you suggest maybe one post-calibration "tweak" to benefit from a longer ride's worth of data? What would be "ideal" for that tweak? I assume a day with little to no wind or change in barometric pressure would be ideal.
>Once you get your device dialed in, I would leave the tweaking alone.
So do you suggest maybe one post-calibration "tweak" to benefit from a longer ride's worth of data? What would be "ideal" for that tweak? I assume a day with little to no wind or change in barometric pressure would be ideal.
Re: Elevation values always negative
The tweaked elevation value is not transmitted to your PP profile. So, if you want elevation "exact" you'll have to adjust it in each of your ride files 
John Hamann