645 Altimeter short changing my stats....

Sawtooth has not been an big issue for me. Last time was in marathon race in first one and half kilometer. Don't know why it happened during thousands of runners started the race.

The manual GPS calibration works if automatic fails. If manual calibrate don't work, just try it again.

The problem is that the elevation is mostly 10-20 meters lower when you end your run activity to the same place (or to the same elevation) where you started the activity. This was the issue in my last marathon race as well - start was few meters above sea level and in the finish line, -8 meters. 

Another problem is that altimeter doesn't show correct altitude change. For example, you run 10-20 meters uphill, but altimeter/elevation shows 2-3 meters only. When you have finished your activity, you may see that the total elevation change is less than half of actual elevation change.

GPS Elevation data screen also shows wrong elevation during the run activity.

Possible workaround when the altimeter does not change although it should: During the run activity when you notice that the elevation does not change anymore (for me usually after few kilometers run), stop running, press stop, wait 20-30 seconds or so, press start and continue running. Now you should see a big difference i.e. the altimeter works nicely again and total elevation is more or less correct after your activity has finished.

Hi, did you ever resolve this issue. My brother got a 645 Music and the exact same thing is happening. For lack of better description the barometric height is changing way slower that what he traverserses. After coming to a standstill after an uphill or downhill ride the barometric altitude takes MINUTES to reach the real level. When plotting height vs. distance it looks like he jumped up to 100 ft up or down while being stationary.

This makes the entire height plot wrong. He got a brand new warranty replacement and that watch does exactly the same thing out of the box. At a complete loss

Nope...I just started always using the "Correct Elevation" feature in Strava after every ride. I've come to accept that the altimeter is a piece of crap.

That’s disappointing. My 945 is spot-on and my brothers 645 does the same thing. He’s moving back to Polar. I had a long discussion with customer support and they “don’t know of anything”. 

Which Firmware do you use, the latest one?

I always let it update to the latest.

Agree 100%. Correct Elevation is the only way I've been able to get accurate accumulated elevation for a ride. Soaking in soapy water helped *a little* but not to my satisfaction.

I can only confirm this. The altimeter is too slow for bike rides or skiing. Works fine for hikes for example because your pace is slow and the altimeter can keep up. I already had one unit replaced under warranty, but this one is the same. I'd like to know if the sensor is the same they use on the 945 or Fenix series or indeed it is a different one, where would I get that info?