system
Given the following post, respond as if you're commenting on Reddit:
human
How do barometers measure altitude accurately when air pressure is constantly changing?

Fitness watches or bike computers - probably some phones too - track altitude change via air pressure change, accurately enough down to the foot/metre.   
How can they do this when the air pressure is always changing.  
And how can they be accurate down to feet/ metres?
gpt
If your question is how is this possible if there is a daily variation of the sea level pressure? Off from the 1013.25 average?

First, the pressure difference between different altitudes is (almost) absolute - so you can always easily measure the vertical speed. By tracking the difference you know by how many meters you moved up or down. The pressure at 0 altitude doesn't matter - only the difference does.

Those that give you an absolute height reading - it is always based off some estimate for the current ambient pressure. Those found in smartphones and GPS-enabled variometers use the GPS the get a rough altitude estimate. GPS has about 10m vertical precision. Then they use it to calibrate the barometric altimeter, which is much more precise. Some are better at doing this, others frequently give erroneous readings unless you manually calibrate them.