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Wrong elevation gain in AUTO Sensor Mode – possible firmware 27.09 bug

gnardella

I've been running some tests on my Forerunner 255 after noticing an anomalous behavior in elevation gain calculation.

Setup:

  • Manual calibration performed
  • Barometer set to Sensor Mode: AUTO
  • The device was kept completely still throughout the entire observation

Theoretical background (from official manual)

According to the Garmin manual, AUTO mode uses both the altimeter and barometer based on detected movement. When stationary, barometric behavior is expected: pressure changes are treated as weather variations, elevation remains stable, and sea-level pressure varies accordingly.

To clarify the two values visible in the sensor screen:

  • Ambient Pressure: raw value measured directly by the sensor
  • Barometric Pressure: sea-level compensated value calculated by the software, based on the calibration elevation

Observed behavior

With the device stationary, ambient atmospheric pressure varies (normal weather variation). I would therefore expect:

  • barometric pressure (at sea level) to vary accordingly
  • elevation gain to remain stable at zero

Instead, I observe the opposite:

  • barometric pressure remains constant
  • elevation gain increases as ambient pressure changes

This behavior is consistent with the device operating in altimeter-only mode: it interprets the meteorological pressure variation as a vertical displacement, recalculates elevation upward, and the sea-level compensation remains artificially stable. This happens while stationary, which is exactly when AUTO mode should not behave this way.

Ruling out hardware failure

If this were a physical sensor issue, raw readings would be erratic and random. The firmware logic of AUTO mode would apply regardless. The fact that a systematic and consistent pattern is observed instead suggests a software problem, not a hardware one.

Hypothesis

I suspect the latest firmware update 27.09 introduced a regression in the movement detection algorithm used by AUTO mode, causing the device to behave like an altimeter even when stationary.

Has anyone else experienced this issue? Can anyone verify the behavior on a previous firmware version?



  • 0

    Mine doesn't behave any differently with 27.09 and auto sensor mode than it did before. It doesn't tend to count ascent in smartwatch mode unless I'm climbing fast enough to count as stair climbing.When I drive around my hilly area, it does not count ascent, I assume because the accelerometer doesn't think I'm walking.

    During activity mode, it is much more sensitive and will count basically all the gentle climbs too. I see this regularly on a walking route I do with about 250 feet of gain over 3 miles. In smartwatch mode it only counts about 50 feet which I think is just one steeper section. This is true whether I use GPS or have it disabled. It seems to just use a different sampling and filtering logic in activity mode?

  • 0

    I am also noticing elevation inaccuracies during activity recording on the latest update. Extra elevation instead of erratic reading. I do not look at elevation readings outside activity recordings.

    I just assumed that the barometer is failing due to the age of the watch - 2 years.

    I thought that submerging it in warm soapy water fixed it, but that has not been the case. Plus I haven't used sunscreen for a while.

  • 0

    Same hier. My FR255 reports ridiculously high elevation gains after a run or a bike ride. FW version 27.09.

    The altitude graphs seem ok, without abnormal altitudes. A bit noisy sometimes, though. Nothing that could explain the very big elevation gain at a glance.

    As an example yesterday my bike ride had around 400m of elevation gain, but the watch shows around 1700m ! The graph is ok, the max altitude is almost exact. And I,ve cleaned the altimeter/barometerhole  multiple times.

    The elevation gain was sometimes funny, but recently it's ALWAYS way too high.

    My old Hammerhead Karoo has no problem at all. Never.

    I won't buy another Garmin with such nasty problems.