There are sensors on the bottom of the aircraft that help it determine it's actual distance between 0 - 30 feet above the ground. According to Autel, some surfaces (such as snow or sand) could affect it's (ultrasonic) ability to determine it's height above the ground (when it is within 30' of the ground). Make sure that the surface that you are taking off from (and landing) is a good, hard surface, and is free of water. And, as always, it's always a good idea to calibrate the compass before you fly, especially when you take off from a new location.