I've owned the Instinct 2 since Christmas and when comparing a walk I do on a regular basis I noticed this week a much larger altitude change than would normally be the case. When looking at the data I noticed large spikes which I had never seen before, see comparison of the walks below.
Walk today:
Zoomed in on spikes:
Previous walk with no spikes:
The difference in elevation between the 2 identical walks is several hundred feet overall as a result of the spikes.
I worked out that these spikes happen typically when I am taking a break whilst climbing the steepest section. This made me think it had something to do with with either pausing the activity or something I was doing whilst stopped. After a quick search on this forum and noticing others mention changing the side on which it is worn it made me think that I could be obstructing the sensor as I typically put my hands on my waist whilst taking a break, with a quick test I managed to confirm this is the case. See video here:
https://www.youtube.com/watch?v=O8EUTVXI1Rw
Immediately prior to this video I had wetted the back of my hand slightly to improve the seal and simulate sweat. My theory is that a combination of sweat and possibly swelling of the hands/wrist create a seal on the sensor port and this creates large pressure fluctuations that the sensor interprets as altitude changes (higher pressure in the sensor is interpreted as a lower elevation). Depending on your wrist shape and now knowing this, you may be able to avoid this issue.
I believe I only just noticed this because now that it is getting warmer I have been sweating more and my wrist/hand has been swelling more than in winter.
I think it will be possible for me to avoid this issue by not bending my wrist back whilst taking a break. For others, it may be their running or cycling style that is causing the issue by repeatedly obstructing the port and creating these fluctuations