In a combined GPS/altimeter device, the calibration and hence the accuracy of barometric altimeter measurements are enhanced with the aid of derived altitudes from a GPS. The device determines the need for calibration and perform the subsequent computations necessary to facilitate the calibration. Furthermore, the device determines a correction quantity that should be applied to barometric altitude readings, thereby allowing the device to be calibrated while in motion. Both of these features ultimately result in a more accurate determination of altitude.