The present invention provides methods and systems related to determine a state of the UAV by updating a determined state of UAV with a relative proportional relationship. The UAV may be provided with a monocular camera, a proximity sensor and a processor. The processor may determine external state information of the UAV based on image data captured by the monocular camera, and calculate a relative proportional relationship to be applied to the determined external state information. The updated external state information of the UAV may be more precise or even equal to the actual state information of the UAV, thus enabling more accurate control and navigation for the autonomous flight.