Sounding line

Depth sounding refers to the act of measuring depth. It is often referred to simply as sounding. Data taken from soundings are used in bathymetry to make maps of the floor of a body of water, and were traditionally shown on nautical charts in fathoms and feet. The National Oceanic and Atmospheric Administration (NOAA), the agency responsible for bathymetric data in the United States, still uses fathoms and feet on nautical charts. In other countries, the International System of Units (metres) has become the standard for measuring depth.

"Sounding" derives from the Old English sund, meaning swimming, water, sea; it is not related to the word sound in the sense of noise or tones.

Traditional terms for soundings are a source for common expressions in the English language, notably "deep six" (a sounding of 6 fathoms). On the Mississippi River in the 1850s, the leadsmen also used old-fashioned words for some of the numbers; for example instead of "two" they would say "twain". Thus when the depth was two fathoms, they would call "by the mark twain!". The American writer Mark Twain, a former river pilot, likely took his pen name from this cry. The term lives on in today's world in echo sounding, the technique of using sonar to measure depth.

A sounding line or lead line is a length of thin rope with a plummet, generally of lead, at its end. Regardless of the actual composition of the plummet, it is still called a "lead." Leads were swung, or cast, by a leadsman, usually standing in the chains of a ship, up against the shrouds.

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