> I'm looking for a good, no-nonsense explanation of the variation of
> pressure with altitude.

> Basically what I want to know is this:  can I use a pressure
> sensor with an absolute range of up to 30 PSI to measure within
> 10 to 20 feet the difference in altitude between the current position
> of a sensor package and the position at which the package was
> initialized?  At altitudes of 1000 feet or less?


1.    Be sure to temperature compensate your pressure sensor if you want 
that sort of accuracy in the real world.
Lots of material available on web on how to do this.

2.    For practical purposes, pressure decreases exponentionally with 
altitude.

Pressure is simply the mass per unit area of the column of gas above that 
area.
For practical purposes you can say that pressure halves for every 14,000 
feet or so of altitude.
So P at 14,000 is about half P at sea level. P at 28,000 feet is about 1/4 
of sealevel. P at 42,000 feet is about 1/8 of sealevel et. Other factors 
come into play, especially at high altitudes where gas temperature changes 
become very significant.

Down low an exponential decay gives you good enough results.


        RM



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