The present invention provides a simple method and apparatus capable of accurately measuring the water depth of a field, particularly, the entire field. SOLUTION: A drone (unmanned aerial vehicle) equipped with an ultrasonic transceiver and an infrared transceiver or a microwave transceiver is flown over a field, and the distance between the ultrasonic wave surface reflection and the microwave or infrared ground reflection is measured. From the difference between the measurements, measure the water depth immediately below the drone. By flying the drone all over the field, the water depth of the entire field can be accurately measured. Preferably, the measurement is made only while the drone is flying above a predetermined speed. [Selection] Figure 2
展开▼