You are correct.  You cannot get both values out of a pressure sensor
unless you hold one of those values constant by not moving or by
stopping the weather changes (ie, getting a report from a local,
stationary pressure at a known altitude and compensating yours with it).

You can get a semi-useful altitude reading from a GPS unit, but they
have much worse altitude error than their location error (ie, +/- 100ft
as opposed to +/- 10ft).

What is the application/acceptable error rate (ie, +/- 1ft/hour is
allowed, etc)?  You can always integrate accelerometers and gyros to
find relative position (dead reckoning).  DGPS and WAAS can correct a
GPS signal so you have better resolution, etc.

-Adam

Gordon Varney wrote:

>I have been asked to through together a barometer and altimeter.
>
>A barometer is easy, using a Motorola MPX4115A. Just convert the voltage out to barometric pressure using a formuli....
>Getting an altitude from this is also easy, Just run another one of them thar formuli.. and you have altitude. The
>problem is, neither one of these measurements are a constant.
>
>If I am driving up a mountain and a storm is approaching then what? The pressure drops from the storm and changes the
>barometric pressure changing the altimeter, which is changing because I am driving up the mountain.
>
>The MPX4115A is calibrated and compensated. Then pressure measured is correct, so how do I know when the change if from
>the altitude or a storm? If I am not moving then no problem. The altitude is the constant, If there is no storm then the
>relative barometric pressure is the constant.
>
>Gordon Varney
>Voice Active Remote Inc.
>www.talk2it.com
>
>
>
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