Calibrating the Altimeter vs Barometer: Are they same thing?

No, they are not the same and both can drift.  It's best to calibrate both.  If you calibrate elevation and the barometer is slightly off, it will change your elevation to an incorrect value once the barometer adjusts because the watch assumes both were correct to begin with.  I'm convinced a lot of the threads you read here about incorrect elevation are due to people not calibrating their barometer too. 

Thanks! For trail running with a total ascent of anywhere between 1000-3000m would you recommend leaving Watch Mode in Auto or Altimeter? Now I don't know if it's because I never calibrated my first Enduro's barometer, it was always inaccurate, worse case was a 34km trail with a total ascent of 2600m +/- 100m, the Enduro recorded 2200m (Watch Mode: Auto) — 600m higher than everyone's watch from that run. I returned it to Garmin, they said "hardware defect" and proceeded with a warranty replacement. 

My personal preference when doing activities with a lot of elevation change was locking the barometer in altimeter mode.  I found this works better because auto mode can incorrectly think the elevation change was due to weather. 

There is no "altimeter" device in the watch at all, how would it work ?, what would it measure ?

Its convenient to think of it as a device or sensor but it isnt.

Your altitude is basically calculated from the changing pressure from the barometer. Say you know your exact altitude, then you set that on your watch and as the pressure rises, the altitude will reduce, as the pressure decreases the altitude will go up. The pressure readings from the barometer are accurate to around +/- 50 feet.

The pressure changes a barometer measures are reasonably constant BUT we dont live in a static environment so you could be standing perfectly still and if the environmental pressure changes because the weather is changing you will see a false change in altitude. So when a storm is on the way and the pressure drops your altitude will go up and vice versa.

Environmental pressure changes with temperature, air density, humidity, and local magnetic variations (although you can forget about that one).

There are various ways to counteract this, so you can calibrate your altitude from:

GPS

This is the most inaccurate way of doing it.

DEM

This can be quite accurate depending on the maps loaded and the source of the DEM data.

Manual (from a known altitude).

This is the most accurate of all, so if you know your altitude, or are at a trig point, or on a map contour line etc. At this point your altitude will be absolutely right.

But for reasons above the altitude may drift so its best to recalibrate whenever you are at a known point, especially if the weather is changing fast.

Garmin gives you various options to calibrate your altimeter at the start of an activity, during etc. Take your choice and see how it goes.

As far as I know there is no way to recalibrate the "raw" barometer sensor at all and there is no need to. When you calibrate the barometer all your are doing is telling it that "the current pressure" represents "this altitude". Its just a pressure sensor and the calibration menu just asks for a GPS, DEM or manual altitude to be entered. You dont get a chance to enter the local QNH pressure setting which would correspond to your altitude above mean sea level (AMSL). On the F7 you will find the altimeter and barometer calibration menus are identical.

You can also set the barometer in 3 modes,

Auto mode, it tries to guess whats happening.

Altimeter mode where all pressure changes will be attributed to you changing altitude in some way.

Barometer mode where all pressure changes are due to weather conditions so the altimeter will be disabled.

At the end of the day as a stand alone device it does its best to give you a reasonable answer but changing weather conditions will always bring in inaccuracies. Its a tool and as long as you understand its limitations its fine to use, just dont expect perfection from a barometric altimeter.

If your activity take 10 or 15 mins, then put it in altimeter mode, it will be accurate.

If your activity is longer over several hours your best bet is to recalibrate the altimeter often when you know your altitude.

BTW every plane has at least 1 barometic altimeter and it works in exactly the same way, and we recalibrate it when  we take off, and when we land and whenever we can as we fly around, sometimes every 10 minutes or so.

Hope this helps a little.

I understand all of this.  What I'm saying both need calibration to remain correct.  For example, years back I was in Vegas walking the strip and the temperature was 108.  Anytime I walked in and out of a casino which had pressurized AC at 72 degrees my elevation would jump up and down to crazy levels eventually leaving me 100's of feet off the actually elevation.  The only way of correcting this would be to recalibrate the altimeter reading to the correct elevation (yes, I too use manual) and then recalibrate the barometer pressure to the correct pressure if it was still off using the msl reported from the nearest weather station.  If I just recalibrate elevation and the barometer hadn't adjusted to the correct pressure when I left the building; say it was off by .13 inHg, the elevation reading would also change to an incorrect level once the barometer corrected itself. 

Coming from aviation.

There is no Altimeter but barometer only.

we need to “calibrate” the barometer and “adjust” the altimeter for the location you are.

We need to have a known altitude ( of a given place) and get a trustable pressure info from a certified station.

e.g. your place is at 800m and a nearby airport station informs 1021mb.

Most of the times when you inform the altitude it will already get very near from the correct pressure but some times doesn’t . Then you calibrate the barometer too.

I use an app called AeroWeather to provide me with a reasonable correct pressure and as for the altitude (when at home) I always use the same one.

Remeber, every single instrument has a error and a variation in the error curve ( not linear).

A little correction to what I said.

If you manually calibrate the barometer, it will then ask you if you want to set the sea level pressure for me in millibars. You can set it as the previous answer suggests from an accurate local weather report, or aviation app etc. For me in the UK its currently 1016.0. This is only really useful if you want to display the current QNH value, useful for pilots, but the altimeter is just run off air pressure changes. So you could set this to 1050 if you like and once you set the altimeter it will still work just as well even though the QNH value is just nonsense.

However if you adjust the altitude then the value the barometer reports will be changed to +/- from its current value to match your altitude change. It makes no practical difference as when the pressure changes the altitude will still go up or down correspondingly. The only reason to set the barometer pressure level is so that you can read a nearly accurate value in a data field. In makes no difference to how the barometer works.