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.
Auto Calibration use (automatically) the GPS to calibrate when you start a new GPS activity. GPS caribration is not so precise, because the GPS system is unable to get the exact altitude, it has an error of about 10 meters. Manual calibration, if you know your current altitude, is the best solution If you need precision.
First some basic stuff about the altimeter and the reasons for calibration: The altimeter calculates the altitude by looking at the difference between the ambient pressure measured by the watch and a known air pressure at sea level altitude. The latter will change all the time with weather and location, so it is not a fixed value. Using a fixed value would give errors which would easily exceed +/-500 meter.
In the watch, the measured ambient pressure is shown as Ambient Pressure, and the pressure at sea level is shown as Barometric Pressure. But for the latter, I will stick to calling it "air pressure at sea level" as I think this makes the explanation easier to follow.
To do an accurate altitude measurement, the altimeter has to know the air pressure at sea level, as it is right now at your location. This is what happens when a pilot just before landing an air plane gets air pressure information from the destination airport so he can calibrate his altimeter to the conditions at the airport.
As a watch user, most often you don't know the air pressure at sea level when you want to calibrate the altimeter. But if you know your exact altitude right now, and you have the measured air pressure at your location, you can calculate the air pressure at sea level yourself and then use this value to calibrate the altimeter. This is exactly what the watch does when you manually calibrate it to a known altitude: You input your altitude, the watch measures the ambient pressure, calculates the sea level air pressure and stores this in the altimeter.
This calibration will drift over time as the ambient pressure at your location changes. So a few hours later, it may be 50 meter off. And then you will need another calibration.
Back to the original question about Auto Calibrate: The behaviour with Auto Calibrate On is probably somewhat different in different watch models with different firmware versions. So your watch may differ from mine (a Fenix 3). But here is what usually happens with Auto Calibrate On:
If you start an activity with GPS, and the GPS has a valid location, the watch will check if you have earlier done a manual calibration at this location. If an earlier calibration is found, the watch will use this altitude to calibrate the altimeter.
If an earlier calibration was not found, the watch will use the current GPS altitude for calibration.
If you leave the activity without pressing Start, the watch will forget the new calibration and roll back to the calibration it used just before. (I don't know the reasoning behind this. In my opinion, it would be better not to roll back.)
Other auto calibration: On top of the above described auto calibration there is another continuously ongoing auto calibration of the altimeter. I don't think this calibration is affected by the Auto Calibrate setting, but I could be wrong.
When the measured air pressure is changing, the watch can choose different strategies for how to interpret it. Did the pressure change because you moved to a different altitude? Or did it change because the air pressure at sea level is changing?
In the first case it would be most correct to keep the pressure at sea level and calculate a new altitude. This is known as Altimeter Mode.
In the second case it would be most correct to keep the altitude and calculate a new pressure at sea level. This is known as Barometer Mode.
My watch will all the time try to guess what is going on and then switch between Altimeter Mode and Barometer Mode as it sees fit. I can't do anything to disable this behaviour (and I wouldn't really want to either). So my altimeter will get small recalibrations all the time whenever the watch decides that Barometer Mode is appropriate. This helps the altimeter to be fairly correct over a few days in most cases, even though I did not do any manual recalibration.
In some watches, and I think that your 935 is one of them, there is a setting where you can choose between Altimeter Mode, Barometer Mode and Auto. With Auto, your watch will behave like I described. The two other options are for forcing the watch into Altimeter Mode or Barometer Mode all the time - which will usually only be the correct choice for some very specific use cases.
Why would one turn Auto Calibrate off: I would only turn it off if I were willing to manually calibrate the altimeter whenever I need it.
I guess alpine skiing where all the lifts have signs showing the altitude could be such a case. But even then I probably wouldn't turn it off, because on a ski trip I have plenty of other things on my mind.
Another special case could be if you are always at the exact same altitude and don't want the watch to change that. Then I would set Auto Calibrate Off, and also force the watch into Barometer Mode (if the watch allows that). For sea sports, this could make sense. (Actually, I belong to that category. I am a sea kayaker, and the watch was bought for this purpose, but still I have Auto Calibrate On all the time.)
On the Edge bike computer you can save a GPS-point - like home - and then set and save the correct elevation for that point. An activity started nearby automatically set the elevation to that reference.
Why is that function not used in the Forerunner series?
Why is that function not used in the Forerunner series?
I don't see anyone claim that it isn't.
But could you explain this is more detail? Will it auto calibrate to any saved GPS location or only those saved GPS locations where you have manually edited the altitude?
But could you explain this is more detail? Will it auto calibrate to any saved GPS location or only those saved GPS locations where you have manually edited the altitude?
Then please tell me why my start point with FR 935 sometimes is different from my saved point with 17 m elevation...?
Any saved GPS location! Idid the same with La Marmotte start area. Saved the location from my house, with correct altitude. Once after start, the Edge wrote in the display "calibrated altitude"...
Just sharing accross forums, as I’ve certainly seen this discussion concurrently before on the 935, Fenix 5 and Fenix 3 forums. All 3 watches work the same for altimeter auto calibration and the best explanation of the behaviour was written by PubBike here https://forums.garmin.com/forum/on-t...68#post1254968
I thought I'd update how I think Auto Calibration of the barometric altimeter (BA) works based on more experience. Note that 8.30 is broken and does not work like this but 8.32 beta is fixed. If Auto Cal. is off, the BA value is unchanged at the start of an activity. With 8.30, the BA value is unchanged at the start of an activity regardless of the setting of Auto Cal.
There are three distinct cases:
Case 1. At a given location, if the BA was previously manually calibrated during an activity at this location, or is now or was previously calibrated using the Settings menu after green ring (GR) at this location - at GR, the BA will be set to the elevation of that calibration (thanks mcbadger for figuring out the Settings menu bit) NOTE: It is not possible to access this list of saved calibrations and it is not generally known how long this list is, but it persists after soft reset (long press LIGHT).
Case 2. If the watch is at a "Saved location" and the BA was not previously calibrated at the current location during an activity or after GR using the Settings menu - at GR the BA is set to the elevation in the "Saved location".
Case 3. If the watch is not at a "Saved location" and the BA was not previously calibrated at the current location during an activity or after GR using the Settings menu - at GR the BA will track the GPS elevation until START is pressed at which point the BA will be calibrated to the GPS elevation at the start of the activity.
All cases: If the activity is started, the BA will be set to the new value. If the activity is cancelled, the BA will revert to the value prior to the green ring appearing unless the calibration was done in the Settings menu during the start of the current activity.
PS: Maybe the reason why this isn't in the manual is because Garmin gave up trying to explain it in remotely simple terms because it is too complicated.
Really interesting and I'll try this with saved locations in my 5x.
To answer one of op's questions, why to turn auto off. I've done it because it not only calibrate before start, it also auto calibrate to GPS at pauses. I've had a 100 m jump during a 10 min pause when not moving at all. The elevation graph and data in file can't be used at all. I'm not sure though if Garmin watch or connect use it for calculation of Elev gain. There are other sites that do it though.
Then please tell me why my start point with FR 935 sometimes is different from my saved point with 17 m elevation...?.
No. I will not.
I have described a method which will work on most Garmin watches with a barometer, and which you could try instead of your Saved Location method: Get a GPS fix, calibrate your altimeter manually. Then the watch will remember that location/altitude combination and use that altitude every time you start a GPS-based activity from that location.
Hey Allan. I have a question about something you said earlier. You said "If you start an activity with GPS, and the GPS has a valid location, the watch will check if you have earlier done a manual calibration at this location. If an earlier calibration is found, the watch will use this altitude to calibrate the altimeter." I did this. I start my runs from my driveway at 470'. Today my watch altitude was reading 499'. I started a run from my saved location and sure enough it was showing my altitude at 470' in my data field without any calibration, but it was showing 470' in the data field for the activity only. For everything else the watch was still showing 499'. Maybe I misunderstood, but I thought you were saying the watch will use this altitude to calibrate the altimeter?
