Depth sounding, often simply calledsounding, is measuring thedepth of abody of water. Data taken from soundings are used inbathymetry to make maps of the floor of a body of water, such as theseabed topography.

Soundings were traditionally shown onnautical charts infathoms and feet. TheNational 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, theInternational System of Units (metres) has become the standard for measuring depth.^[1]

"Sounding" derives from theOld Englishsund, meaning "swimming, water, sea"; it is not related to the wordsound in the sense of noise or tones,^[2] but tosound, a geographical term.

Traditional terms for soundings are a source for common expressions in the English language, notably "deep six" (a sounding of 6 fathoms). On theMississippi 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 writerMark Twain, a former riverpilot, likely took hispen name from this cry. The term lives on in today's world inecho sounding, the technique of usingsonar to measure depth.^[3]

Asounding line orlead line is a length of thinrope with aplummet, generally oflead, at its end. Regardless of the actual composition of the plummet, it is still called a "lead", often referred to asthe lead. Leads were swung, or cast, by a leadsman, usually standing in thechains of a ship, up against theshrouds.^[4]

Measuring the depth of water by lead and line dates back to ancient civilization. It continues in widespread use today in recreational boating and as an alternative to electronicecho sounding devices.Greek andRoman navigators are known to have used sounding leads, some of which have been uncovered by archaeologists. Sounding by lead and line continued throughout themedieval andearly modern periods and is still commonly used today. TheBible describes lead and line sounding inActs 27:28, whilst theBayeux Tapestry documents the use of a sounding lead duringWilliam the Conqueror's 1066 landing inEngland. Lead and line sounding operates alongside sounding poles, and/orecho sounding devices particularly when navigating in shallower waters and on rivers.^[3]

At sea, in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it is common practice to tie marks at intervals along the line. These marks are made ofleather,calico,serge and other materials, and so shaped and attached that it is possible to "read" them by eye during the day or by feel at night. Traditionally the marks were at every second or third fathom: 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, while if it was between two marks, he would call "by the deep" followed by the estimated number; thus "by the mark five", since there is a five-fathom mark, but "by the deep six", since there is no six-fathom mark.^[a][3][7] Fractions would be called out by preceding the number with the phrases "and a half", "and a quarter", or "a quarter less"; thus 4 3/4 fathoms would be called as "a quarter less five", 3 1/2 as "and a half three", and so on.^[7] Where the depth was greater than 20 fathoms the line would commonly be marked with a single knot at 25 fathoms, two knots at 30, and so on.^[8]

Soundings may also be taken to establish the ship's position as an aid innavigation, not merely for safety. Soundings of this type were usually taken using leads that had a wad oftallow in a concavity at the bottom of the plummet. The tallow would bring up part of the bottom sediment (sand, pebbles, clay, shells) and allow the ship's officers to better estimate their position by providing information useful forpilotage andanchoring. If the plummet came up clean, it meant the bottom was rock. Nautical charts provide information about the seabed materials at particular locations.^[3] Nautical charts also include depth contour lines. It is thus sometimes possible to navigate in poor visibility by noting which contour line one is closest to.

During the nineteenth century, a number of attempts were made to mechanise depth sounding. Designs ranged from complex brass machines to relatively simple pulley systems. Navies around the world, particularly theRoyal Navy in Britain, were concerned about the reliability of lead and line sounding. The introduction of new machines was understood as a way to introduce standardised practices for sounding in a period in which naval discipline was of great concern.^[9]

One of the most widely adopted sounding machines was developed in 1802 by Edward Massey, a clockmaker fromStaffordshire. The machine was designed to be fixed to a sounding lead and line. It featured a rotor which turned a dial as the lead sank to the sea floor. On striking the sea floor, the rotor would lock. Massey's sounding machine could then be hauled in and the depth could be read off the dials in fathoms. By 1811, the Royal Navy had purchased 1,750 of these devices: one for every ship in commission during theNapoleonic Wars.^[10] TheBoard of Longitude was instrumental in convincing the Royal Navy to adopt Massey's machine.^[9]

Massey's was not the only sounding machine adopted during the nineteenth century. The Royal Navy also purchased a number of Peter Burt's buoy and nipper device. This machine was quite different from Massey's. It consisted of an inflatable canvas bag (thebuoy) and a spring-loaded wooden pulley block (the nipper) through which the sounding line was passed. Again, the device was designed to operate alongside a lead and line. In this case, the buoy would be pulled behind the ship and the line threaded through the pulley. The lead could then be released. The buoy ensured that the lead fell perpendicular to the sea floor even when the ship was moving. The spring-loaded pulley would then catch the rope when the lead hit the sea bed, ensuring an accurate reading of the depth.^[9]

Both Massey and Burt's machines were designed to operate in relatively shallow waters (up to 150 fathoms). With the growth of seabed telegraphy in the later nineteenth century, new machines were introduced to measure much greater depths of water. The most widely adopted deep-sea sounding machine in the nineteenth century wasKelvin's sounding machine, designed byWilliam Thomson (Lord Kelvin) and patented in 1876. This operated on the same principle as lead and line sounding. In this case, the line consisted of a drum of piano wire whilst the lead was of a much greater weight. Later versions of Kelvin's machine also featured a motorised drum in order to facilitate the winding and unwinding of the line. These devices also featured a dial which recorded the length of line let out.^[11]

Both lead-and-line technology and sounding machines were used during the twentieth century, but by the twenty-first,echo sounding has increasingly displaced both of those methods. A sounding line can still be found on many vessels as a backup to electronic depth sounding in the event of malfunction.GPS has largely replaced the sextant and chronometer to establish one's position at sea, but many mariners still carry a sextant and chronometer as a backup. Many small craft still rely solely on a sounding line.

The first practicalfathometer (literally "fathom measurer"), which determined water depth by measuring the time required for an echo to return from a high-pitched sound sent through the water and reflected from the sea floor, was invented byHerbert Grove Dorsey and patented in 1928.^[12]

• Depth gauge – Instrument that indicates depth below a reference surface
• Echo sounding – Measuring the depth of water by transmitting sound waves into water and timing the return
  • Multibeam echosounder – Type of sonar used to map the seabed
• Fishfinder – Sonar instrument used to detect aquatic animals and the bottom and measure their depth
• Challenger expedition – Oceanographic research expedition (1872–1876)

• Media related toDepth sounding devices at Wikimedia Commons
• The Lead Line – Construction and use (retrieved Sept 2006).

• Navigation
• Hydrography
• Earth observation
• Vertical position

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