Abstract: A method and apparatus for automatically determining a characteristic of a crop using an unmanned aerial vehicle (UAV) includes operating the UAV at an elevation above a ground surface of a field and collecting first data that indicates an elevation of the UAV with a barometric pressure based altimeter of the UAV. Second data is collected with a SONAR sensor of the UAV that indicates a distance to a nearest object on the ground surface. The SONAR sensor has a SONAR sensor range and the elevation above the ground surface is less than the SONAR sensor range. The method further includes receiving the first data and the second data on a processor, and determining a height of the nearest object above the ground surface with the processor based on the first data and second data. Output data that indicates the height of the nearest object is presented on a display.