Re: Most accurate altimeter wrist watch?

You have to look it up (e.g. a road sign that gives the altitude). Or
barring that, you can measure relative changes - you can guess at your
starting altitude and then know how far you have ascended/descended from
your guesstimated starting point (assuming that the weather has not changed
in the meantime).

These watches are really barometers. Barometric pressure is a function of
altitude plus fluctuation due to weather (forgetting temperature for the
moment). So you have an equation with two unknowns - if you know one you can
solve for the other, but if you don't know either you can't fully solve the
equation.


If your "altimeter" is showing that you have "ascended" it could be because
you really are at a higher altitude or because a low pressure system has
come thru since you last calibrated the watch - if you have moved from your
starting point you don't know which and you can't in the absence of outside
reference. However, since weather is relatively constant over a short
period, you can assume that all changes over the course of a day are due to
actual ascent from your starting point and you will be approximately right.


BTW, the barometric pressures that you see printed in your local newspaper,
etc. are not true pressures but are corrected to subtract the effect of
altitude. For example, "normal" pressure is given as 29.92" of mercury but
at 5000 ft the actual pressure on an average day is only 24.9" (because the
column of air pressing on the barometer is 5000 feet shorter and weighs much
less). If you read in the newspaper that day that the pressure is 30.12",
the true pressure that day is 25.1" but such a low pressure would indicate
a storm beyond all belief and confuse people so they give a normalized
pressure instead. The barometer on your watch does not read the newspaper or
know about your actual altitude - all it can do is read off the true
pressure and it is up to you to supply the missing information.

For true altitude without reference to a known pressure or starting altitude
you could use a GPS receiver. If a GPS receiver has reference to 3
satellites it can triangulate your position in 2 dimensions (i.e. latitude
and longitude). If it has reference to 4 satellites it can also position
you in 3 dimensional space (i.e. altitude as well).


"Mij Adyaw" <mijadyaw@xxxxxxxxxx> wrote in message
news:IseYe.141451$Ji4.10181@xxxxxxxxxxxxx
> That is the issue that really sucks with altimeter watches. How do you
> know the local pressure?
>
> "John S." <hjsjms@xxxxxx> wrote in message
> news:1127310915.128941.316230@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> none wrote:
>>> Can anyone recommend a wrist watch with a reasonably accurate altimeter
>>> function?
>>>
>>> Thanks,
>>> -Mike
>>
>> Don't forget to re-calibrate the altimeter to the local pressure
>> reading before ascending.
>>
>
>


.

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