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Altimeter calibration questions

SpikySimonW
over 4 years ago

Recap:

  • Device uses built-in barometer to measure pressure, from which it can reasonably accurately measure altitude gain/loss.. or a weather system change (the trick being to differentiate between the two).
  • Calibrating at the start of a run (as an example), assuming accurate, would give the correct starting elevation (less important to most of us) and from there the barometer records relative altitude gain/loss (more important to most of us).
  • Setting the device to continuously auto-calibrate during activities does just that, the purpose being to keep in check any pressure changes/drift not due to altitude change but weather systems, so should improve accuracy.. but again relies on the auto-calibration getting good values.
  • When manually calibrating, you can either enter the current altitude (most accurate if you know this), use DEM (built into the maps, requires a 2D GPS fix, generally will be accurate although averaged out over a small area) or use GPS (requires good 3D GPS fix, has the potential to be most accurate for your exact location, as well as the potential to be most inaccurate due to GPS errors!).
  • While pressure changes due to weather systems can occur anywhere anytime, the likelihood of this is obviously reduced on say a 5-10km looped run around a park (20-50 minutes) vs. a point-to-point ultra trail run lasting 7+ hours.

Questions:

  1. When auto-calibration is set, either “once at start” or “continuously”, is it the DEM or GPS method that is used?
  2. Also when the GPS method is used, is DEM referred to in order to filter out obvious outlier readings?
  3. Is the time elapsed time/distance from start taken into account at all when auto-calibration is set (my "short run round a field" vs "long ultra marathon" point)? Or is this something the user should take into consideration when setting up their watch / before the start of specific activities?
  4. Finally, how does the “Not during activities” auto-calibration work? A lot of this time the watch might be indoors, therefore without being able to get a GPS fix..?

TIA. :)

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  • 0 over 4 years ago
    1. It uses DEM (because it is generally more accurate than GPS; GPS is very accurate on the X-Y plane, but has terrible Z-axis accuracy)
    2. Nope. It uses the reading even if it is way off from the DEM. This is a good thing, actually, because the DEM file isn't perfect either, and if DEM calibration doesn't work then it's good to have a fallback that's totally independent from it. Generally speaking, the most accurate method will always be entering a known elevation manually, next best is DEM file, 3rd best is GPS. If you don't know the elevation, and DEM file is giving you a calibration that doesn't make sense, then use GPS calibration as your fallback. Also, DEM file will not work if you're significantly above ground level (on a bridge, tall building, flying), whereas GPS calibration will work at any height AGL.
    3. Nope - continuous auto-calibration takes your barometer altitude reading, and your DEM altitude (based on current GPS x,y coords), and if there's a difference, then it drifts your elevation reading gradually (not instantly) until it matches the DEM altitude. This can be seen if you're in an airplane, and have continuous calibration turned on. As you fly through the air at a fixed altitude, the altimeter on the watch will gradually descend (over the course of several minutes) until it eventually shows you at ground level, which is where the DEM file thinks you are.
    4. This one is interesting. It only calibrates once per day, sometime during your sleep window. It uses your phone's GPS location (rather than the watch's, because it is assumed your phone has more accurate and up-to-date location data at that point) and the watch's DEM file. Unfortunately, when you use the "Battery Saver Mode During Sleep" setting on the watch, this calibration never happens because the watch turns off BT during the window when it's supposed to be getting GPS coords from your phone. I've been complaining about this for months but unfortunately it still doesn't work. No big deal, I honestly just do a manual calbration once every week or two and it's fine.
  • Former Member
    0 Former Member over 4 years ago in reply to C.sco

    Well said and perfect answers.  One thing I'd like to add is DEM is reliant on GPS.  If you're next to a ridge, hilltop or saddle and GPS has you slightly off to the side one of these terrain features, then the DEM will be off also.  If you're on a ridge and it has a 50 foot drop off, GPS may show you below the ridge and DEM will be off. It's always best to use DEM in an area where the terrain is the same to avoid this from happening. 

  • 0 over 4 years ago in reply to C.sco
    C.sco said:
    because the watch turns off BT during the window when it's supposed to be getting GPS coords from your phone.

    Edit the Battery Saving power mode so that the Phone stays connected - change from Turn Off to Don't Change.

  • 0 over 4 years ago

    For the correct measurement of altitude difference D+ during an activity which is the best setting?

    Not during activities or auto-calibration?

    thank you

  • 0 over 4 years ago in reply to maxcalce

    "Not during activities" is somehow a strange setting (for me): it only calibrates once a day during the night the altitude and a bt connection to phone/gcm is needed for that.

  • 0 over 4 years ago in reply to maxcalce

    I found the auto-calibration setting terribly off. 

    They say it has been improved in last RC firmware 16.30, but I haven't tried it yet.