New Fenix Pro 6s - Elevation Incorrect?

You can calibrate it manually through the altimeter widget or by using GPS or DEM. If you know the elevation, manual calibration is better. In the settings section of the altimeter widget, you can also set it to auto calibrate at the start of an activity or continually. I set it to calibrate at the start since it can get wonky if I'm running through an area with tall buildings. I've found the altimeter to be pretty accurate. However, during the hot season when I get really sweaty, it can cause it to spike. In that case, I'll sometimes change it to continuous calibration.

Due to the nature of a barometric altimeter, you'll have to calibrate it periodically.

Definitely worth doing a manual calibration of a brand new watch.

Sensors and Accessories> Altimeter> Calibrate

As you know your current elevation, I'd use that method over DEM or GPS.

It's also worth saving your home (or whereever you normally start your runs from) as a location, once you have calibrated your current elevation.

Thanks guys, appreciate that, i'll calibrate it from a known elevation today. 

I'll also save some key points where I typically run from as 'locations', i'm assuming it is then simpler or quicker for the watch then to pick up GPS and elevation from 'memory'?

your run profile elevation data comes from the barometric altimeter, not GPS.  While its possible to display the GPS based altitude information on the screen, what gets saved to the profile file is from the baro altimeter.  Looking at your weather today the reported barometric pressure is 29.01in, which would explain why you're seeing baro altitudes in the 300m range instead of 18m.  Standard baro pressure is 29.92in, and lower baro pressure translates into higher altitude readings.  A difference of 0.91in is about 900-1000 ft of elevation, so everything sounds normal, you just need to recalibrate the baro altimeter back to 18m before each run, because the baro pressure constantly changes.

Hhhmmm thanks for that explanation, i didn't appreciate any of that.

If that's the case it's frustrating I've just shelled out £450 to upgrade to a watch that i thought would resolve this issue instead of always having to re-calibrate.

I saw a similar issue with a 645 Music that I had briefly, just assumed it was a rubbish altimeter like VA3, so probably that watch would have been fine for £200!

altimeters are very misunderstood by the average user, I guess that's why they try to put auto-calibration features into the mix.  FWIW, I've never had an altimeter that didn't perform accurately, but I know exactly how they're designed and operate, so I know when the bad reading is due to some external influence, and not the altimeter itself.  I'd say the chances are quite good that your other altimeter was working just fine.

The VA3 one was completely haywire, like a circular run would start at 10m rise up to 50m then finish (at the starting point) at -30m, Garmin admitted it was an issue. 

As for the 645 music, yes I flipped and took it back after I saw my first run was -150m.... however this Fenix 6 wasn't any better at +250m. In hindsight it was probably fine.

The most reliable and simple tracker I've ever had (and still have) is the Vivosmart HR+, it just seems to work relatively well with never any issues. 

I'm beginning to believe my physio who thinks technology creates more stress/tension/anxiety to perform and instead get a £10 casio with a stopwatch and go by feel.  

I have mine configured to auto-calibrate automatically (it does it once per night, while I'm asleep, using the phone's GPS location), as well as auto-calibrate at the start of each activity (but not continuously during the activity).

I've had very good results with that combination of settings. Very rarely do I have to do a manual calibration, and even when I do (maybe once or twice a month), it's usually only to correct it by less than 50 feet. 99% of the time, my altimeter is accurate to within 25 feet or so.

For example, I haven't done a manual calibration in weeks, and currently my watchface says I'm at -5 feet. My actual elevation is about +10 feet. So after several weeks of not manually calibrating it, it's only off by 15 feet. Quite good accuracy if you ask me.

So I would recommend using the auto-calibrate (outside of activity) and calibrate at start of activity settings, don't use continuous calibration, give it a try for a few days, then come back and report here if you still don't think it's working right.