The easiest way to develop an altimeter is by measuring directly the atmospheric pressure (see an501.pdf "Using MS5534 for altimeters and barometers"). Atmospheric pressure represents in fact the weight of the air above the pressure sensor.
Since the amount of air present above the sensor decreases when the altitude increases, we can modelize the relationship between pressure and altitude.

This application note provides a simplified model of the relationship between the pressure and the temperature so that it can easily be implemented in an 8-bit microcontroller.

The MS5534 or the MS5540 have a resolution of 0.1 mbar representing 1 meter of altitude (when near the sea level). The wide operating range (10 mbar to 1100 mbar) makes them suitable for any application from the sea level up to the top of the Mount Everest.

The MS5534 / MS5540 products from Intersema are then the ideal choice for that application.

Thanks to their digital interface, they can be directly connected to any microcontroller. The internal calibration PROM is factory programmed allowing high accurate measurements.

For more details, follow these links: MS5534 / MS5540.