Depth Sounding
Depth sounding refers to a historical nautical term for measuring depth; it is often referred to simply as sounding. Sounding is finding the depth of a given point in a body of water. Sounding data is used in bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...

 to make maps of the floor of a body of water. Soundings were traditionally given on nautical charts in fathoms. The National Oceanographic and Atmospheric Administration (NOAA), the agency responsible for bathymetric data in the United States still uses feet on charts, but metres have become the international standard for measuring depth. Sounding was originally done by hand with sounding poles or a weighted sounding line
Sounding line
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."...

 for greater depths.

Sometimes soundings were taken to establish position, a navigation function then, rather than safety alone. Soundings of this type were usually taken using tallow coated leads with a big wad of tallow
Tallow is a rendered form of beef or mutton fat, processed from suet. It is solid at room temperature. Unlike suet, tallow can be stored for extended periods without the need for refrigeration to prevent decomposition, provided it is kept in an airtight container to prevent oxidation.In industry,...

 in the bottom concavity. The tallow would bring up part of the bottom sediment (sand, pebbles, clay, shells, etc.) and allow the ship's officers to better estimate their position.

Traditional terms for soundings are a source for several important common expressions in the English language, notably "deep six
Deep Six
Deep Six may refer to:*Deep Six , a 1986 compilation album*Deep Six , a Dirk Pitt adventure novel by Clive Cussler*Deep Six , a group of fiscally conservative members of the Legislative Assembly of Alberta, Canada...

" (a sounding of 6 fathoms) and Mark Twain
Mark Twain
Samuel Langhorne Clemens , better known by his pen name Mark Twain, was an American author and humorist...

 (from "by the mark, twain", for a 2 fathoms). The term lives on in today's world in echo sounding
Echo sounding
Echo sounding is the technique of using sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes...

, the technique of using sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

 to measure depth. See Fishfinder
A fishfinder is an instrument used to locate fish underwater by detecting reflected pulses of sound energy, as in SONAR. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris,...

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