Garmin Fenix 3 - Temperature spikes result in abnormal elevation changes

Question

Oscillating temperature readings result in inaccurate altitude data. I currently have no solution or explanation. Can anybody help?

In this case the route is completely flat. As soon as the temperature starts oscillating (exactly where I placed the cursor) the altitude increases. This a new, unexpected, behavior. Something I am seeing only recently...

Another more extreme example is here. In particular temperature spikes result in abnormal elevation changes.

Here is a normal behavior of the temperature/elevation metric, it taken same time of the year almost one year ago, similar temperature conditions.

Answer 1

I would expect an aneroid or electronic barometer to do exactly that. Without any compensation, anything that affects pressure will yield changing readings. Be it pressure differentials caused by being on the leeward or windward slopes of a ridge in high winds, or temperature changes due to time of day or exposure to sunlight.

If it is a new behavior, maybe another system in your barometer is failing and there is no compensation being applied. Or maybe you simply have changed the operating mode. Many barometers will have the ability to switch between fixed elevation, displaying pressure changes similar to a weather station, and varying elevation, used as an altimeter.

Considering your temperature changes chart, which shows oscillations of 2-3°C, this is very consistent with what I experience in my Garmin cycling GPS unit (Edge 510) that includes a barometer. Depending if I'm in the shadow or in the sun, there will be similar temperature changes.

Garmin has a page about some of the behaviors described for a specific watch (but this applies to any barometer, really). Here's a National Geographic article on how barometers work, which should be helpful.

Answer 2

The thing is that altimeters are affected by the air temperature because hot air will cause the pressure to rise and cold air to cause the pressure to drop.

Warm air causes air pressure to rise. When air molecules collide, they exert force on each other. When gas molecules are heated, the molecules move more quickly, and the increased velocity causes more collisions. As a result, more force is exerted on each molecule and air pressure increases.

...

Cool temperatures cause air pressure to drop. When gas molecules cool, they move more slowly. Decreased velocity results in fewer collisions between molecules and air pressure decreases.

How Does Temperature Affect Barometric Pressure?

So as the air temperature fluctuates, the altitude shown on your device will change as well. This can even be used to predict weather with the altimeter.