Definition
Sound speed in seawater rises with temperature, salinity and pressure, ranging roughly from 1450 to 1550 metres per second. Survey grade sounders apply a measured profile because refraction bends beams and an incorrect assumed velocity biases depths, an error of about 15 metres per second producing roughly one percent depth error. Small craft sounders simply assume a fixed value, which is adequate for pilotage but not for charting quality data.
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