WHR and Barometer hugely inaccurate

My 5x altimeter has always been off. The altitude of my home foundation is 25 feet. Two days ago, standing outside, the GPS calibrated it to 43 feet (not too bad for GPS). But today it reads 446 ft. I have not figured out how to manually calibrate it to a value lower than its present reading. Seems the manual calibration only allows me to enter an integer for the "thousands of feet" positon. Each calibration only remains for a day or two, then it changes to some random number, sometimes negative, or a thousand feet to high. Was useless when hiking in the mountains. Online manual is no help at all.

Haven't paid much attention to the barometer reading.

Overhere, on my forerunner 935 the same. 

My watch tells me the the airpressure rose from 1052 to 1055 in the last 6 hours. No other values outside this range. 

My altitude however, had a sudden drop from about 380 meters to 350 meters and back to 386 meters. The low part lasted for about 1 hour. 

My actual altitude is sealevel. 

The actual airpressure at the moment is 1004

But I couldn’t bother anymore. Garmin only adds features for marketing purposes and don’t seem to care if they are usefull or work at all. 

Can’t blame them. People chose Garmin because the (seem to) have a longer featurelist than the competition. And it works. Garmin sells more devices than Suunto and Polar. 

The Barometer doesn't report altitude, that is the Altimeter.  Which are you looking at?

Altitude is either reported by gps, but this is not very accurate or calculated by airpressure. (airpressure drops with altitude)

Airpressure is very accurate if the barometer is good enough and weather doesn’t change. 

Most recreative airsports, like hot airbolloons, hang gliding, ultra lights, use airpressure for altitude