Navigation General · ECDIS & Electronic Aids
An echo sounder determines water depth by:
- AMeasuring the change in hydrostatic pressure at the transducer face
- BTransmitting a sound pulse downward and measuring the two-way travel time to the seabed✓ Correct
- CComparing the GPS altitude to a stored bathymetric database
- DMeasuring the resonant frequency of the water column between keel and bottom
Explanation
An echo sounder (depth finder) transmits a pulse of sound (typically 200 kHz for shallow water, 50 kHz for deeper water) and measures the time until the echo returns from the bottom. Depth = (sound velocity × travel time) / 2. The division by 2 accounts for the two-way path. Sound velocity in seawater is approximately 1500 m/s.
Authority: Bowditch (Pub. 9), Ch. 14
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