Calibrate local barometer

who knows said:

The barometer on your watch calibrates pressure at your location based off msl.  You can add a data field that shows ambient pressure and there's a widget in the IQ store that shows both if ambient pressure is more important to you. 

First, you calibrate instruments, not quantities. So you calibrate a barometer, not pressures. 

This is a pedantic but important distinction, because the "barometer calibration" of the watch is not a calibration at all, given that it does not adjust the readings of the instrument, rather it changes the values of a derivation of it (sea level pressure which is a combination of local pressure and altitude).

That said, it's not just that "ambient pressure is more important to me". If the ambient pressure is wrong because the barometer is calibrated badly, then it affects altitude readings as well. That may be the source of the problems I'm having with the altimetry on this watch, always underestimating elevations.

Interesting. I just tried and, while the local pressure currently reads 990.7, setting the altitude at 0 it gave me 989.66 as "sea level pressure", basically the same offset I'm experiencing. 

Mmmm... food for thoughts.

Sea level and local would display as the same with 0 altitude. Checking on my 945 (my 6 isn't handy right now) local pressure is displayed to 1 decimal place and sea level calibration pressure is entered to 2 decimal places. Perhaps that's the reason your local is slightly higher (rounded up)? It's a shame there is no way to add a calibration offset directly in the operation system though with these devices that level of precision isn't really necessary, it's the trend that's more important

You could do this but it will be a real pain. For example if you know that to get your garmin and your reference barometer to agree you needed to add +2m then whenever you calibrated your altimeter you would add an extra +2m to what you were expecting. Say I was calibrating from a spot height on a map and that was 100m then I would actually enter 102m into the watch. That would hopefully remove any “error”. As said earlier though any calibration offset may not be consistent at different altitudes. At see level you may need +2 but at 2000m it may be -5. Would need to do some testing and I'm not sure it’s actually worth all the hassle with a device like this!

No, as you said correctly there is no guarantee that the offset would be constant at different elevations. All in all I don't think it is worth the hassle, but thanks for your thoughts!

Clatecnofilo said:

After, I've read most of the comment, I didn't get the conclusion. Is the watch barometer accurate and working correctly or not?

Thank you for trying! I value every opinion, so I will try to explain the issue better.

In the Fenix, you can "calibrate" the altimeter and the barometer. Actually, only the altimeter part can be calibrated directly: if you know your elevation, you can enter it into the watch. The barometer calibration, instead, is not a real calibration: you don't set something you measure directly, as the reading of Fenix barometer, rather you set the pressure that you would read if you were at sea level.

This is conceptually wrong, because you should be able to calibrate the barometer on board the Fenix directly, set the altitude, then calculate the sea level pressure.

Next picture is taken at my laboratory, comparing the pressure read by the watch with that read by a several thousand euro calibrated pressure reference:

The difference, 0.7 hPa, at this elevation (~ 250 m) corresponds to roughly 5 m. It is not negligible at all. Tramplite is correct in saying that the actual pressure should not be that important, as long as the trend is correct: if the measured pressure is 991, then I go to a place where the reading is 998, then I know I moved ~ 50 m lower: as long as I calibrated the altimeter correctly, it should give you the correct elevation of the end point.

I suspect, though, that the difference between the reading of the watch and the actual pressure value plays a role in the inaccuracy I'm experiencing with the elevation readings of the watch. That's why I would like to calibrate the actual barometer, instead adjusting the values of sea level pressure.

Garmin documentation states:

With proper calibration, the barometric altimeter of an outdoor watch or handheld will report elevation readings ranging from -2,000 to 30,000 feet within +/-50 feet of accuracy. Elevation values greater than 30,000 feet can be generated, but may not be accurate due to environmental factors. 

5m is +/- 16.4 feet. Well within the Garmin stated +/- 50 feet. So actually its doing pretty well.

Calibrating the barometer wont change its accuracy it will still be accurate to +/- 50 feet.

It just means at the instant you calibrate it, will may read the correct pressure. And even then its pretty meaningless as ISA pressure is defined with a standard temperature and humidity etc etc.

Pressure readings on weather forecasts all over the world are given so that your altimeter will read your altitude above mean sea level.

https://en.wikipedia.org/wiki/International_Standard_Atmosphere

If you want to put a strap on the desktop device and carry it around with you, please do, but what you are wearing is a watch, that's all, a pretty amazing watch, but still a watch and the tiny little barometer inside has limitations, and as your measurents show,  its well within the stated accuracy.

Thank you for your answer.

The problem is, I'm having a lot of issues with Fenix altimetry, much higher than the stated accurary by Garmin. I've had 200 and 300 m of error during hikes. And I'm trying to understand what is the problem.

I still can't get why my eTrex 30 is so vastly superior to the Fenix when it comes to elevation. Few days ago i went for a hike in the mountains, with both Fenix and eTrex. I started walking at 1900 m, and when I came to a pass at 2706 m the eTrex read - guess what - 2706 m, while the Fenix was reading 2669. 37 m of difference (~ 120 ft), way beyond the accuracy stated by Garmin. I recalibrated the Fenix there, and at the top of the mountain, the eTrex still gave the exact measurement to the meter, while Fenix already was lagging 15 m behind.

I really don't think eTrex has a better barometer: it was built 12 years ago, I refuse to think that in 12 years the quality of the sensors has gone down so much.

gspinoza said:

The problem is, I'm having a lot of issues with Fenix altimetry, much higher than the stated accurary by Garmin. I've had 200 and 300 m of error during hikes. And I'm trying to understand what is the problem.

I still can't get why my eTrex 30 is so vastly superior to the Fenix when it comes to elevation. Few days ago i went for a hike in the mountains, with both Fenix and eTrex. I started walking at 1900 m, and when I came to a pass at 2706 m the eTrex read - guess what - 2706 m, while the Fenix was reading 2669. 37 m of difference (~ 120 ft), way beyond the accuracy stated by Garmin. I recalibrated the Fenix there, and at the top of the mountain, the eTrex still gave the exact measurement to the meter, while Fenix already was lagging 15 m behind.

For what it's worth I also see a larger drift in altitude compared to my F3 when comparing start and finish altitude for a 1 hour run. 

I have watch mode set to Auto and calibrate altitude at start of the activity, not continuous.

Regarding the pressure and calibration you can check if it is the barometer that is wrong or the algorithm to detect elevation changes over time that is wrong as long as your eTrex can show the ambient pressure. If you have the same difference at start between your eTrex and F6 as you have on the top the barometer is working. Even if it is not calibrated to perfect accuracy it should still work.