I’m getting watch face altimeter readings with a minus sign preceding the reading. Readings like -276 feet. What does that mean?
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If an uncorrected barometric altimeter is reading significantly less than the true altitude then that means that the local sea level atmospheric pressure is greater than the standard pressure of 1013bar. That is, if you are standing at or near sea level and the sea level pressure is much higher than 1013bar then your watch would say that the altitude is -ve.
A smaller error can also be due to non-standard current sea level air temperature and non-standard temperature variation with altitude, including for example if there is a temperature inversion layer.
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Michael…I really don’t want to belabor this subject. It’s very obvious that you know much more than I’ll ever know about altimeter readings. I just don’t know why developers include this feature in their watchfaces if you’re trying to reason the readings and you are not meteorologist. My biggest problem with this is I’m a man of 77 years old and need things simplified. I would like to look at an altimeter reading and have an inkling of what I should understand.
Thank you for your patience!
John L
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Hey John, all good - happy to chat about altimeters … 
Mostly I think having a barometric altimeter on a watch face is just sort of a cool novelty. Climbers and others who want to know differences in altitude over a short period of time would no doubt find them useful but otherwise it is just as easy to look up Google Earth if you want to know the altitude of a particular spot.
But it is possible to correct one of these altimeters by entering in the current sea level atmospheric pressure and temperature - well, mostly correct them anyway. I thought you might have synced one of my corrected altimeters some time ago ( but then again I might be confusing you with someone else ).
Really I think they are mostly just a bit of fun and they do look cool! At least mine do 
Of course, the errors can be very large, way too large to be used safely for any kind of safety critical task such as navigation.