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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 finding the depth of a given point in a body of water. Sounding data is used in bathymetry 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 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 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 2 fathoms). The term lives on in today's world in echo sounding, the technique of using sonar to measure depth.

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." Leads were swung, or cast, by a leadsman, usually standing in the chains of a ship, up against the shrouds.

Sounding lines were widely used in navigation until the development of echo sounding. Ultrasonic depth sounders provide an accurate graphical profile of the depth of the seabed.

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

Sounding technique

It is easy to measure a length of line or rope as a rough number of fathoms by repeatedly stretching the rope between the two outstretched arms. Water depths have traditionally been measured this way using a weighted sounding line. The word fathom can be used as a verb to describe this process.

At sea, in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it became traditional to tie marks at intervals along the line. These marks were made of leather, calico, serge and other materials, and so shaped and attached that it was possible to "read" them on sight by day or at night by the feel of each one. The marks were at every second or third fathom, in a traditional order: at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. The "leadsman" called out the depth as he read it off the line. If the depth was at a mark he would call "by the mark" followed by the number, if the depth was between two marks, he would call "by the deep" followed by the estimated number.

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 there were only two fathoms left under the boat they would call "by the mark twain!". The American writer Mark Twain, a former river pilot, likely took his pen name from this cry.

Sometimes tallow was placed at the recess in the bottom of the plummet to pick up traces of any loose material (such as mud, sand, or shingle) from the seabed, providing information useful for pilotage and anchoring. If the plummet came up clean, it meant the bottom was rock. Nautical charts now provide information of the seabed materials at particular locations.

See also

• Echo sounding
• Fishfinder

References

1. Sounding Pole to Sea Beam, NOAA, http://www.history.noaa.gov/stories_tales/poletobeam.html
2. Kemp (ed.). The Oxford Companion to Ships and the Sea. p. 150.
3. "Sound, v". Oxford English Dictionary (Second ed.). Oxford, England: Oxford University Press. 1969.

• P. Kemp, ed., The Oxford Companion to Ships and the Sea (London: Oxford University Press, 1976).

External links

• An explanation of the fathom marks used at sea (retrieved Sept 2006).

• Navigation