Smart Altimeter uses pressure sensor on your device to measure your altitude above sea level or previously chosen reference altitude (if your device does not have a barometer, the app will fall back to GPS). Advanced multiple-layer algorithms are used to smooth noisy measurements and to calculate descent rate. The app can also speak your current altitude. Use of devices with high-speed barometric sensors (60Hz or faster) is highly recommended for the feedback lag not to exceed 0.2s (the lag is 12 samples).