Large Altimeter Corrections during runs

I already had the problem, I had to recalibrate the altimeter so that it was good afterwards. I don't know if it's a crappy GPS point that gave it a false altitude or something else, but I already had the problem. It is very rare in any case.

You went through a pressure front is my opinion.  I've seen the same thing on a flat track.  Mine and her watches showed a quick climb of a 250 feet, stayed there for a couple of hours then dropped back down for the rest of the run.  I looked through weather data and sure enough a low pressure cell as part of area storms came through at that time. 

The altimeter is based on atmospheric pressure which while in a steady state world would be 100% constant, the world isn't in a steady state with wind patterns and such and does shift and change as low and high pressure cells get created and dissolved.  

The same bad problem on two days and you really think it is a pressure front?

If you are pausing the activity and then resuming it later, make sure your watch acquires a good GPS signal again before you resume the activity. This will ensure the watch gets an accurate elevation point to resume the activity at which should resolve the issue.

This seems like it is likely the DEM map (digital elevation model) that is used for altitude calibration, although odd it happens on both devices.  The first thing I'd do is try turning auto calibration off during a run (set it to "off" rather than "continuous" or "at start") and see what happens.  That would tell you if it's some weird barometric pressure thing or cached elevation point.  I doubt it's either of those since it happened on two devices on two separate days.

You could always try forcing an update of the maps for your region using Garmin Express.  This would also update the DEM maps.

Also, I've never seen a pressure system that would cause a 600' drop.  And I can't imagine GPS would cause such a dramatic change either.  That is a huge drop.

ekutter-dnu That's a good point, Continuous auto calibration will use the DEM map and GPS elevation so turning that off should let you know if there is an issue with the mapping. When you connect your watch to Garmin Express it should show a specific update to the DEM mapping if there is one.

If you don't already have Garmin Express you can download it at this link: https://www.garmin.com/en-US/software/express/windows/

Even if it shows you're up-to-date on the maps, it's worth telling it to re-install anyway.  I believe it's a single download, but then gets split into regular map and DEM map in your Garmin folder.  You should have a couple DEM maps there.  The one with a "1" at the end of the name is actually the golf DEM map and only has info for golf courses.  Make sure you have plenty of time because installing the maps is a slow process.

@Garmin-Nate talking about altimeter... when this issue will be solved? Just read last 3 or 4 posts... https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/172029/projected-elevation-plot-when-navigating-an-auto-route-is-it-correct-on-the-f6-looks-like-not-fixed

Garmin-Nate said:

If you are pausing the activity and then resuming it later, make sure your watch acquires a good GPS signal again before you resume the activity. This will ensure the watch gets an accurate elevation point to resume the activity at which should resolve the issue.

The watches were both on AutoPause- and yes, the elevation jumps coincide with run pauses. It has been doing it on every run since SW 8.10 update (today's as well) I did have my watch set on AutoCal during activity.  So, I've just changed that to only "at start".

Maybe the other thing is... I shouldn't stop in the middle of my runs  :)  

The one thing it definitely is NOT.... is a pressure front.