①The sound velocity of water changes; because the sound velocity of water varies according to the turbidity inside.

      ②The change of water temperature; the sound velocity of water is different under different water temperature. When the temperature in the whole water area is constant, temperature compensation can correct part of the measurement error caused by the change of water temperature.

      ③The surface of the measured object: Because the ultrasonic emission has a certain angle, and the surface of the underwater measured object (such as river, silt, rock, sunken ship, etc.) is not a flat surface, the accuracy of the depth sounder cannot be improved. The echo sounder can only measure the time of transmission and reception, and cannot be sure whether it hits a flat surface.