Snow depth is measured using a sonic ranging depth sensor, which is shown in the image to the above. This sensor sends a sonic (sound) signal from its base to the surface below. This signal is then reflected from the surface to the sensor where an air temperature-calibrated distance-to-surface measurement is calculated. This distance is then subtracted from the initial height of the sensor above the ground to yield a snow depth measurement. The accuracy of this sensor is +/- 1 cm (approximately 0.3937 in.). Each snow depth measurement undergoes a quality control procedure that checks for instrumentation error and data integrity according to the manufacturer's recommendations.