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Depth sounding refers to a historical nautical term for measuring depth; it is often referred to simply as sounding. Sounding is the act of determining the depth of a given point in a body of water. Sounding data is used to in bathymetry to create 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 fathoms on charts, but meters have become the international standard for measuring depth. Sounding was originally done by hand with sounding poles or a weighted sounding line 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 (a soft waxlike semi-sticky substance) 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" ( a sounding of 6 fathoms) and Mark Twain (from "by the mark, twain", for a 2 fathoms). The term lives on in today's world in echo sounding, the technique of using sonar to measure depth. See Fishfinder (fathometer).
References
- Sounding Pole to Sea Beam, NOAA, http://www.history.noaa.gov/stories_tales/poletobeam.html