Calibrate local barometer

gspinoza said:

2- I really don't think that raw GPS of Fenix is more similar to eTrex, given that the latter used baro AND GPS info as well,

The e-trex baro data is going to be very similar to the Fenix baro data.  They're both calibrated to the same standard atmosphere, and both will suffer the same inaccuracies as a result. So, given that, the big question is how does the e-trex do so well measuring altitude, when its baro altimeter suffers the same errors?

One possibility is the e-trex GPS tends to read high, and gets balanced out by the baro altimeter always reading low.  Or, maybe the e-trex puts a very small weighting on the baro data, and relies more heavily on the GPS.  Otherwise, if the e-trex was using its baro elevation data with any significant weighting, you'd still see elevation errors, but not quite as large an error as you see with the Fenix that logs the baro data weighted 100%.  

I think to get to the bottom of this you need to see the raw GPS elevation and raw baro altimeter data for each device and compare them.  I'd be surprised if there's much difference between the two if you have a clear reception area for GPS all the time. 

The few times I've used the Fenix GPS and looked at the altitude it reported, it was within a few feet (at several different altitudes from sea level up to 2500m in the mountains).   Based on that small sample set I have no reason to believe its not accurate.  I use the baro altimeter all the time, and know it to be quite accurate as well, given the constraints of the standard atmosphere.

On one of the other threads someone suggested that putting the watch into barometer mode, where all pressure changes are assumed to be due to weather, forces the logging activity to use the GPS elevation data instead of the baro altimeter elevation data.  Not sure if it will actually force that, but I'm curious if you've ever tried that method.

5693709 said:

On one of the other threads someone suggested that putting the watch into barometer mode, where all pressure changes are assumed to be due to weather, forces the logging activity to use the GPS elevation data instead of the baro altimeter elevation data.  Not sure if it will actually force that, but I'm curious if you've ever tried that method.

Interesting suggestion, no I didn't try.

Yesterday I went for a hike with eTrex and Fenix, putting in the latter GPS raw elevation and PP elevation side by side, and comparing to the elevation on eTrex. I can't say I learnt much, given that the hike was in a narrow canyon and even the eTrex struggled.

It seemed that PP elevation was always greater than GPS raw by 20-40 m, and that the latter was somewhat similar to eTrex GPS. Fenix PP elevation, in the end, was 15 m higher than in the beginning (same spot) but I think that's not bad given the poor signal reception in the area. eTrex end-beginning elevation was 40 m, because the elevation was wrong in the beginning right after the satellite fix.

All in all, I think I will have to do more tests in more open area.

Just an update on using the GPS altitude on my Fenix, at the end of a hike yesterday the actual altitude was about 8110ft, the Fenix GPS read 7980ft.  There were many trees in the area, so not the best reception conditions.  My cell phone GPS reported 8045ft, closer, but not perfect either.

So, my earlier comments that I've only used the Fenix GPS sparingly and found it reasonably accurate the few times I used it is amended based on this use, which I would not categorize as particularly accurate.  But, this was also the most challenging reception conditions I've encountered as well, I usually have wide open sky.

Maybe this question was simmilar as yours. https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/262745/fenix-6-pro-factory-barometric-pressure-incorrect
We cannot calibrate or change the pressure sensor which inside in the watch. I had done some experiments: use a mercurial barometer to test the air pressure compare with fenix under the same condition(both in a windtight box). However the pressure value are different. Thus I think the pressure senor inside in fenix has different algorithm with others.  

Thank you Master_Ming, yes I read that thread but it seemed to go nowhere...

More than a different algorithm, I think the sensor was incorrectly calibrated in the factory, or it was calibrated correctly but since then it drifted.

I was just surprised it cannot be calibrated by the user.

To be honest, I had try to use different digital barometer to test if their values are same. The fact is No, even same sensor type. It's a very strange phenomenon.

Well commercial barometers may be badly calibrated. I see that all the time in my work. 

I used a top-level, accurately calibrated barometer as a reference, and the values were different by 1 hPa, which is a lot.

Hi,

I recently opened this topic too. Also working in calibration laboratory and did some test, even changing the pressure in pressure chamber. The result is 2.1hPa offset. Please try to wrote to Garmin with sugestions link to add a ambient pressure offset possibility as I did. Maybe we could change that :-).

www.garmin.com/.../

forums.garmin.com/.../f7-pressure-laboratory-compare-just-for-fun