Altitude drops every 1-2 km of each run

Are you finishing at your starting point?

Yes, almost always if it isn't a commute bikeride.

Here is the complete graph from yesterday's run.. From house to the park, couple of laps, and back homeciq.forums.garmin.com/.../1362164.png

Perhaps wait a little longer after opening the activity and before pressing the start button, to allow the GPS elevation to calibrate the barometric altimeter.

I actually already do that, about a minute before i head out the door, to get a GPS lock, before i start my run

Some of the newer Garmin devices including the Fenix 5 seem to have a temperature dependent altimeter error. I guess that is what happened to you.

When surrounding temperature is changed, it will take the watch approximately 15 minutes to be fully heated up / cooled down. You ran at 5:10 minutes/km, and it took 3 kilometers before the altitude curve leveled out. That fits the 15 minutes very well.

I actually already do that, about a minute before i head out the door, to get a GPS lock, before i start my run

Could you try changing the order to> Head out the door, get a GPS lock, wait one minute?

I am afraid he will have to wait 15 minutes instead of 1 minute, so the temperature of the watch will settle.

Also, to remove the initial altitude error, which is probably an old, memorized altitude for that location, he will probably have to do a manual altitude calibration at that location once, while GPS is on.

Some of the newer Garmin devices including the Fenix 5 seem to have a temperature dependent altimeter error. I guess that is what happened to you.

When surrounding temperature is changed, it will take the watch approximately 15 minutes to be fully heated up / cooled down. You ran at 5:10 minutes/km, and it took 3 kilometers before the altitude curve leveled out. That fits the 15 minutes very well.

That would make sense, here is the temp graph. Any way to disable that without wasting 15 min on not working out? :Pciq.forums.garmin.com/.../1362558.png

It is not possible to disable the barometric altimeter.

You could complain to Garmin that your altimeter is temperature sensitive. It shouldn't be. But I don't know if you would get any results out of that.

I am wondering if this is an error in the ambient pressure reading, or a calculation/assumption error in the conversion from ambient pressure to altitude. It is not entirely unthinkable that some developer thought he was clever and thought that he should correct the calculation for air density variations caused by ambient temperature - in which case your example would prove him wrong.

Could I persuade you to make a custom screen in an activity with the following four values:

• Ambient pressure
  
• Barometric pressure
  
• Elevation
  
• Temperature
  

Then report the values you see immediately after going out and the values you see when the temperature has settled.