Altimeter/Barometer is extremely inaccurate

I noticed this same, but I did  not sure if it is Altimeter/Barometer because some activities what I recorded was aproximately 20 - 80meters out of reality (check it with smartphone and also via maps).

I think bad Altitude may be due to not precise GPS chip in watch.

I think it would be difficult for the watch to autocalibrate to compensate for high and low pressure due to weather. The watch does not know when you are outside and can receive a signal. There is however a feature to calibrate time by using GPS (I have put it in my circular quick access menu). Maybe this feature calibrates the pressure sensors as well? 

The watch doesn't know whether the weather or your altitude is changing, and in watches like the VA4, it's in an auto mode, deciding on whether it thinks changes in pressure are due to altitude change, weather change or temperature change. 

On some of the more expensive watches like the Fenix lines, you've got the options to set your elevation at a known elevation, and also change other features that allow the watch to make different assumptions... 

See Garmin's link HERE

On a VA4, it's not unheard of or unexpected to need a daily calibration. There is a lot of information regarding barometers, barometric readings, and how barometric altimeters work. I just came up with the conclusion that it needs a lot of user input and manual calibration.

GPS altimeter calibration can also be very inaccurate at times. If I wait 30 seconds with GPS, my doorstep altitude is 35m according to GPS. Sometimes, on GPS+Glonass, (satellite dependant as the watch chooses either GPS or Glonass depending on reception) can be 20m. If I wait up to a minute or more with GPS only, it comes to 19m. 

The watch should get the more "advanced" feature for the user to set a known altitude.  It's frustrating otherwise. 

A few days? The weather can change enough in half a day to offset the altitude by more than 100 meters. It is physics, not a bug in the watch or the pressure sensor. You need GPS calibration every hour or so to keep the barometric altitude correct.

Well, that's the thing though...I am not too bothered by the altitude being wrong, but by the barometric Sensor showing the wrong pressure, often by a significant margin.

The barometric number that gets shown is usually the sea level corrected pressure and the raw value.

The sea level correction value I don't think is ever perfectly accurate, at least not in my experience across airports and weather stations... they all have slightly different sea level corrected values. This is somewhat expected. I can look at 6 different station MSLP readings and they'll all be different. Just like a barometer on a wall at home, you need to calibrate it to a value based off a nearby weather station... based off that, you can determine trends. The accuracy of this calibration value isn't that important as far as I understand... its the interpretation of the trends that tell you whether you are going up or going down in pressure, and thus getting an idea about how the weather will change. Alone, this value also doesn't mean much either, and temperature and humidity need to be factored in too.

If I want to keep track of the weather, and determine if there might be a strong afternoon wind, then I need to keep an eye on the pressure trend my watch is showing... irrelevant if its calibrated to 1013 or 1020... if I see the pressure decreasing, I know I'm in for a windy afternoon around here.

to quote Wikipedia;

The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level (PMSL). This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure adjusted to sea level, not the actual local atmospheric pressure.

Perhaps there are some developers that can input here about what info Garmin allows them to access when it comes to barometer numbers. Temperature changes and altitude changes will affect the barometric reading. Simple. You cannot go on a hike and notice your barometer reading is going down and then expect that to only dictate bad weather... when you are rising in altitude, the weather and your skin temp may be rising, and a low pressure zone has developed in your area. 

I'm not an expert, and I would like to be corrected if I'm wrong, but this is how I understand barometers. Its all relative. 

Thing is...my Vivoactive seems to have an "upward creep" on the Pressure level. It starts around 1000 and within a few days arrives at 1100 or more. This happens pretty regularly. Weather stations and my phone's barometric sensor never actually go that high...

OK. Thats substantial. 

Have you tied a factory reset? I can't say it'll fix it but its worth a try. At the cost of having to set up your device from scratch. 

When my watch was unboxed and was bombarded with multiple updates, everything went haywire. My altimeter defaulted to -490m, and it needed more than 40sec to calibrate to a reasonable altitude. Also, a high rate of battery drain.

The factory reset helped fix it for me. 

Since my watch is working perfectly apart from this one issue, I haven't done that yet. Maybe I will do that at some point, if it turns out to be a real problem for me.

For now, I might just adjust my watchface to omit the Pressure reading...