I was looking at Garmin's product support page for the 935. While reading about elevation, the website explained the "auto", "altimeter" and "barometer" mode settings in quite detail. In the section about calibrating the altimeter it reads "From here you can choose Calibrate to calibrate the sensor using a current known elevation or by using the GPS elevation reading, or toggle the Auto Calibrate setting on or off." Calibrating it manually with a known elevation or using GPS are self explanatory, but I can't find any info on auto calibration. Would anyone be able to explain what the auto calibration "on" does and how it does it? Secondly, why is there an option to turn it off? Is there a situation where one would want it on as opposed to off or vise versa? Any help with this would be appreciated.
I get tired of reading this. Sorry. People need to learn that air pressure can change, also when the weather doesn't change.
This is why both GPS and BP altitude have their place in the mountains, and why it would be nice to have a setting where it recalibrates over time if the GPS and BP altitude diverge. The GPS altitude is always more reliable as you are approaching a summit. The BP altitude is going to be better at telling you how much vertical you've gotten, especially in valleys. Doesn't Suunto effectively have a continuous auto-calibrate mode?
I'm finding that my FR935 has been showing gradually increasing elevations over time. I'm at ~5300 feet altitude but at one point my watch (at the same location) was showing well over 6,000 feet elevation. My watch is set to Auto, but I did do a manual calibration w/ GPS about a month ago (back to Auto afterwards). After that, the elevation has subsequently been increasing again on average about 100 feet a week (always up not down). I understand and accept that the elevation shown by the watch will vary over time due to pressure changes, etc., and I'm not that concerned about having a 'precise' elevation, but it seems odd to have to do a manual calibration every month or so to return the watch to an expected value.
If you understand that the elevation reading will change when the air pressure changes, how can you think that a calibration once per month should be enough?
The air pressure will change much faster than that. You can easily get elevation drift of 30-50 feet/hour due to a changed air pressure at your location.
A daily calibration would be more appropriate if you want any accuracy.
Is it possible that the GPS fix for yesterday and today was so far off that the watch didn't recognize that I was at my usual starting point? I guess I'll just have to manually calibrate every activity until I can figure out what's going on.
Yes. You have to be within approx. 25 meter of the location with a known altitude. So if you got a bad GPS fix which put you more than 25 meter away, you may not have triggered the auto calibration to the stored altitude.
However, in this case I would have expected it to auto calibrate to the current GPS altitude. Did you wait for Green Ring (assuming that the 935 has the Green Ring)?
Anyway, it may be a good idea to have a data screen with two fields, showing the GPS Elevation and the Elevation data fields. If you switch to this data screen when starting an activity, you can see what is going on. This will make troubleshooting easier.
This is why both GPS and BP altitude have their place in the mountains, and why it would be nice to have a setting where it recalibrates over time if the GPS and BP altitude diverge. The GPS altitude is always more reliable as you are approaching a summit. The BP altitude is going to be better at telling you how much vertical you've gotten, especially in valleys. Doesn't Suunto effectively have a continuous auto-calibrate mode?
It would be nice to have a continuous auto-calibration of the barometric altimeter based on continuous comparison of the barometric altimeter and the GPS elevation, yes. But only if it is done right.
I can easily think of implementations that I would really hate. For example auto calibration to the current GPS altitude every x minutes (which I think some Garmin watches used to do).
Suunto has something called "fused altitude" which does some kind of continuous auto calibration or combination of the two altitude signals. I don't know their algorithm, so I don't know if I would love or hate it.
As a ex owner (2 years) of Ambit 3 Peak i can assure you that it works very well. I do a lot of hiking and trail running and the altitude on the watch was always correct, or very close (~10m off). On the other hand, 935 elevation (baro) always is ~100m off, while the gps elevation (i have a data field with that) is accurate.
This keeps coming up but I don't really understand the fascination with Fusedalti, it's just a software trick. Better to use your brains and have a page with both the Baro and GPS altitude that way you can get a sense of the pressure/weather changes, assuming the GPS altitude is correct (and in the mountains with no tree cover it generally is). I'll be skiing next week so will keep an eye on this.
What's difficult about getting that FusedAlti, generally, just works? If one can come to rely on it through experience that it's accurate, why would you want to look at the barometer and GPS derived altitude screens then wonder which one is correct?
I explained above why you'd want to display both at the same time, that shouldn't be too difficult to "GET" either surely ? GPS altitude IS correct, barring any catastrophic environments, the only problem is that it changes a lot so messes up D+/D-, hence the inclusion of barometer sensors on GPS watches, and of course they're useful to anticipate/understand trends in the weather.
It is a shame Garmin takes such a simplistic approach to elevation estimation when solutions exist which are much better (e.g. FusedAlti), as it seems anyone who has performed activities in the mountains will have found that the GPS is generally pretty good whilst the barometric altitude gets progressively further from the truth. As a result you can reach a summit with watch saying you are 100m lower. As a result the total ascent and descent will be poor estimates.
There could be a very simple easy to understand option (unlike Auto Cal!) that automatically calibrated the barometric altimeter to the long term (e.g. 5-10 minutes) smoothed GPS elevation every 30 minutes say. Both the smoothing time constant and the time interval between calibrations could be selectable parameters, but even with fixed values decided by Garmin (that could be different for different sports to account for speed) it would be better than the current system.
I could imagine a couple of settings within Auto-Cal 1) At start of activity: on/off (performed as today using the list of previous calibrations and saved locations to perform the calibration) 2) During activity: on/off, calibration interval (minutes), smoothing time (minutes) (performed using the long-term smoothed GPS elevation)
Less useful but nevetheless helpful alternatives might be: - make Auto Cal work when the activity is resumed after pausing - provide an option on the activity save menu to recalibrate to GPS elevation when the activity is paused - make Auto Cal work with Resume Later so that to recalibrate you pause the activity, Resume Later, and then restart the activity
...which made me think of a really cumbersome workaround: For hiking and cycle touring, save an activity every 30 minutes and then splice the fit files together afterwards.
