Hey, I just got one of these too. A Christmas present from my girlfriend, bless her for knowing what a gadget freak I am… Cool, aren’t they.

Anyway I have been reading up on this subject a bit lately, so I know whereof I speak. Or type.

The watch has a pressure sensor that measures the current absolute pressure.

Say it is 995 millibars. (You might be using inches of mercury as your units, but the principle is the same.)

You check your local weather report and find that the pressure is, say, 1004 millibars. The weather stations correct their readings for altitude, so what they are giving is the **sea level pressure**.

So, if you tell the watch that the sea level pressure is 1004 millibars, and it’s sensor tells it that the local pressure is 995 millibars, it thinks “Hey, the pressure is 9 millibars less than at sea level… let’s plug that into the ol’ height equation” and will come up with (rough guess… 1 millibar = 30 feet height difference) a height of 90 feet. Or whatever.

If you are interested, the equation is:

z = (RT/gM).log[sub]e/sub

where z is the height difference between the starting height and the measurement height, R is the gas constant, T is temperature of the air in degrees Kelvin, g is the acceleration due to gravity, M is the molar mass of the gas (in this case air), p[sub]o[/sub] is the atmospheric pressure at the starting height and p is the atmospheric pressure at the measurement height.

This is affected somewhat by the temperature of the air - the greater the difference between actual temperature and “standard temperature” at a given height, the greater the difference in altitude will be.

I don’t know if you have the same watch as me - mine is a Suunto Observer, and it gives an altitude readout to 1 metre… enough to track your progress as you walk up and down stairs. It seems to be unbelievably accurate too, once calibrated. Obviously you need to do this regularly, as changes in air pressure due to weather systems will affect your apparent altitude.