>Relative barometric pressure is the constant.
> >
> > That's why airports have barometers so that pilots can compensate their
> > alitmeter to the pressure at a known altitude for the location they care
> > about.
>
>         On can always go GPS, get a fix on at least four sats and the GPS
> spits out
>current altitude, quite accurate AFAIK. TTYL

I just went flying with a friend who had a new gps with the map. He was
hitting on 5 satellites (I think)
and the altitude and airspeed were bang on what the instruments were
saying.  Very impressive.
And here I am with flying around my 20 year 50 cent esso road map......;-]

Dave

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