Focus on the offset not the negative - this seems to be less than 20m out so not too bad.
Have you tried a calibration within the Sensors & Accessories section?
Well it´s the firts time that I have a altimeter in my watch. 610, 620, 630, 735XT... and now I did as you explained and I calibrated the altimeter using a online orto map to fetch my current altitude.
Just to throw in some map nerd questions - what are you comparing to? The sea goes up and down, and usually isn't at "sea level". GPS uses a geoid model to determine position - are you using this same model to compare against? Local mapping will often use a custom datum, which is usually not WGS84 and therefore may use a different definition of "sea level" or "0 height". As Tim said, 20m is pretty close. After calibration you may then see your altitude go down if pressure goes up and that could easily be 20m difference, unfortunately these sensors are not perfect :)
Just to throw in some map nerd questions - what are you comparing to? The sea goes up and down, and usually isn't at "sea level". GPS uses a geoid model to determine position - are you using this same model to compare against? Local mapping will often use a custom datum, which is usually not WGS84 and therefore may use a different definition of "sea level" or "0 height". As Tim said, 20m is pretty close. After calibration you may then see your altitude go down if pressure goes up and that could easily be 20m difference, unfortunately these sensors are not perfect :)
Used this two pages, the altitude is the same were I an at the moment. 95 meters.
