lagombike

From an airport close to my place I get current barometric pressure 1002,9 mBar. This value is the "QNH"-value, e.g. the for me unknown value that was noted at the airport has been been recalculated to be correct as if measured at the mean sea level. In my Garmin Etrex Vista Hcx there are two pressures present, the barometric and the ambient. When calibrating the altimeter by putting in a mBar value it is, as I understand, the barometric value that is the correct input. The difference between the barometric value (mean sea value) and the ambient value gives the altitute above meas sea level of the Garmin gadget. I have found out that I must add 1,4 mBar to this value, the barometric airport value (1002,9 mBar) and the new corrected value is 1004,3 mBar. After I have entered the corrected value in Garmin Etrex Vista Hcx I receive the altitude to 20 meter which is the correct altitude for the Garmin above the mean sea level (when placed on my kitchen table). The accuracy specified is plus minus 10 feet (ca plus minus 3 meter) but 1,4 mBar is around 11 meter. Has this needed correction been noted by someone else or discussed earlier? Cheers! ... and Happy New Year !!