The last few sessions I have done have had huge changes in altitude. All been pancake flat.
is there something that you have to do to get this reliable? Any suggestions Garmin-Maeve or Garmin-Sierra
two sessions today showing it as an example. One at the start had the jump, and the second hours later had a massive drop at the end.
I had exactly that on my out-the-box-new FR970. I did any and all testing I could, down to a factory reset, but no avail, had to conclude it was a faulty sensor. I contacted garmin support and they opened…
Got the exact same issues with my 970 straight out of the box.
This didn't work for me. After trying everything else I gave this a try this morning with a manual calibration at +21m wich is the exact height outside my house. Then started my run hoping for the best…
So it ended up being a hardware problem like angelakru. After the SW updates not changing anything I contacted Garmin again to let them know the problem persisted. They than replied with the question for permission to look into my connect account for the data during runs. After that I got send an email saying that the problems where caused by a defective altimeter sensor. I have now received a replacement 970 and this one works like a charm. Hopefully this one keeps working properly 
