GPS altitude is wrong during an activity - influenced by air pressure?

Hi, I am living in a totally flat area near the coast. My verified altitude at home is 28 meters.
When I start an activity, the watch uses GPS to read in the correct value: 28 meters.

I noticed several times that when I do a longer acivity, let's say a 3 hrs hike or bike tour, the watch then shows 42 meters when I am back at the exact same place where I started from, instead of the correct 28 meters.
Air pressure was rising in this case.

Now this isn't correct, as it should always show 28meters at the same position, as the watch during any activity always gets the altitude from GPS.

I have not found any possibility to fix this. I also contacted Garmin about it today and they offered no solution.
Did you notice something similar too?

ciq.forums.garmin.com/.../1344983.jpg

0 AllanOlesen67 over 7 years ago

Your watch does not use the altitude from GPS during an activity. It only uses the altitude from the GPS once at the start of the activity to calibrate the barometric altimeter, and then the barometric altimeter is used during the activity.

If the watch was using GPS altitude instead of the barometric altimeter during the activity, you would get a lot of false ascent meters because of noise on the GPS altitude. The barometric altimeter can drift a little during the activity when the surrounding air pressure changes, but it has almost no altitude noise.

So you got 14 meters of false ascent because of air pressure drift, but if you had used GPS altitude for 3 hours, you would probably have had 10 times as many meters of false ascent.

0 gaijin over 7 years ago

Your watch does not use the altitude from GPS during an activity. It only uses the altitude from the GPS once at the start of the activity to calibrate the barometric altimeter, and then the barometric altimeter is used during the activity.

If the watch was using GPS altitude instead of the barometric altimeter during the activity, you would get a lot of false ascent meters because of noise on the GPS altitude. The barometric altimeter can drift a little during the activity when the surrounding air pressure changes, but it has almost no altitude noise.

So you got 14 meters of false ascent because of air pressure drift, but if you had used GPS altitude for 3 hours, you would probably have had 10 times as many meters of false ascent.

A perfectly well stated, factual and accurate response - well done, sir!

0 abennett over 7 years ago

I usually turn on my 5X in flights. You see there the huge difference, GPS altitude is often over 10000M and barometric is 1500-2000 (inside aircraft pressure)

0 AllanOlesen67 over 7 years ago

I usually turn on my 5X in flights. You see there the huge difference, GPS altitude is often over 10000M and barometric is 1500-2000 (inside aircraft pressure)

And...?

Since you already indicate that you are aware that the pressures inside and outside the aircraft are different, it should not be any surprise that the barometric altimeter shows a wrong value inside the aircraft. I can't see any relevance to the topic being discussed here.

0 razmichael over 7 years ago

not to mention ... if, at 10,000M my barometric altitude showed the same as my GPS I suspect there would be other things to worry about!:)

0 Former Member over 7 years ago

There is a well known issue with altimeter calculations in the whole Fenix branch that was never addressed and has nothing to do with variations in air pressure or temperature.
Unless someone here wants to claim that if you walk from 150m to 1m and then walk back to 150m, those 10-12 m difference depend on a variation that happens in 20 minutes time, without a single cloud in the sky and no wind.
Sure, the variations can be significant during a 3 hour ride but no chance in 20 minutes.

Luckily this kind of problem is not so significantly changing the accuracy of overall data, but the problem is there, has always been there and I don't think it's the baro so it has to be the software.