Forerunner 955 altimeter not reliable

Pavel_T: the weather and pressure difference in two different times is understandable. But it does not explain when I do the run wearing both my watches at the same time.

1st picture: my Garmin on a 6.2k run just finished an hour ago, running out and back, starting and arriving the same spot.

2nd picture: my Coros on the exact same run, same time. I think the difference is quite straightforward.

Also, during a 40 minutes run there cannot be that much pressure change unless a big storm is coming which does not happen every day.

I would not complain if the Garmin would not cost twice as much...

Cheers!

Hello,

Sorry in advance if my english is not the best.

i have the same issue with my Garmin Forerunner 955 (non solar) with Firmware 26.08. I bought the watch in April 2024(1year 8months). Never noticed major issues with the altimeter before, but recently in my runs i am seeing strange numbers. Because of a Achilles Injury i am avoiding hills so my runs are basically on flat terrain and i live by the ocean. So that means i would expect that a mininum elevation of 0meter and max elevation of 25meters on the runs i am doing. Of course can be a margin of error that i would expect to be something like around 15 meters (Garmin states 10 feet/3 meters).

"With proper calibration, the barometric altimeter of a Garmin watch or outdoor handheld will report elevation readings ranging from -2,000 to 30,000 feet with an accuracy of +/-10 feet at any given point."

But in recent run is like this:

8/12/2025 6,32km

Total Ascent 21m

Total Descent 60m

Mininum Elevation -36m

Maximum Elevation 14m

10/12/2025 8,03km

Total Ascent 42m

Total Descent 92m

Mininum Elevation -48m

Maximum Elevation 31m

The calibration is on auto, but nevertheless before starting a run i calibrate manually to 8meters (the known elevation of the place where i usually start the runs). After starting the activity the alimeter usually starts to rise until something like 15-30m and then at some point goes to values below 0, but not inside the margin of error. 

Its like the sensor its acting really sensitive to temperature changes (at least i suspect about that).

Another examples: my home in the third floor of a building so the altimeter should read something like 15meter. I caliber the altimer manually to that number. If there is no temperature changes usually the fluctuations are minimal. But if i go do bed, under the sheets during the night the temperature rises and the altimer starts to record numbers like -16m. Or if i go to the window in a sunny day, like it happened today, if i stay there for some minutes, there it goes from 15m to -31m as the temperature went from 26degrees to 33degrees. My wife has also a garmin foreruner 955 non solar and on this test i had it on my other wrist, and the altimer stayed put on the 15m. The difference is that her watch is 1year1month old and is on firmware 24.12 (mine is 26.08).

Just to add that i dont swim on pools or open waters, just used it once on a lake last year in the summer. And the sensor ir not clogged since i remove the skin cells/sweat with tap water after most of the runs.

Maybe this beaviour could be from software? Can Garmin please check this on a next update please?

Best Regards,

Tiago Oliveira

Perfect analysis. I'm also convinced it's a problem with temperature variations. The problem is hardware because I have friends with the same firmware as my 26.8 who have no problems Mine was replaced under warranty and the new one had the same defect again after a few months. 

Found it. Total different figures when I change it. The manual at garmin tells me the tiny hole for barometric sensor needs to be free of sweat and dirt. But it seems located at the bottom of the watch, at the back. So it is in contact with my skin (and sweat). I wonder if that is influencing the readings... 

Yesterday i did a road trip and passing by several towns i compared the number on the altimeter with the known values of the places and everything was just fine. In the way back everything was still spot on, same values in every town till a reached my home town. The temperature inside the car was constant or with little variation, its difficult to not suspect about the temperature to have a significant role when crazy altimeter readings show up. Other thing that could be at play is the sweat, but since this wasnt a activity and i was just sitting down inside the car nothing bad happened. In the next run i am going to wear the watch on top of a sleeve, maybe that way i can rule out the sweat of the list of suspicious factors.

Meanwhile, today i did another run, this is what happened:

21/12/2025 9,05km

Total Ascent 43m

Total Descent 112m

Mininum Elevation -46m

Maximum Elevation 37m

A friend of mine did the same run with me, he haves a Garmin Epix Pro Gen 2:

Total Ascent 51m

Total Descent 45m

Mininum Elevation 3m

Maximum Elevation 25m

To be honest i found this a little bit frustrating.

It's surely related with temperature, mine during summer gives more accurate altitude than in winter

I think this discussion is going off on a bit of a tangent.

Yes, there is clearly a temperature compensation issue here which isn't done properly and can result in the elevation gain over a typical run being off by a few tens of metres. Yes that's probably a fault, but an error of that size could also be explained by not having adequately cleaned out the barometer port or something like this (instructions for this are available online).

But the fault I and some other people have seen with the elevation on these devices is much more dramatic than this with the elevation sometimes being off by as much as a factor of ten! I've attached a few screenshots of runs as they were recorded in my Garmin and then from strava where the actual elevation gains have been DEM-corrected to a more realistic figure. One run was recorded as 381m, was actually 54m:

Another was recorded as 412m, was actually 135m

And this one, a nice flat coastal run, was recorded as 278m when it was actually no more than 60m.

Until I got the watch exchanged, _every single run_ was showing these sorts of errors! This was emphatically NOT just a temperature compensation issue!