Editing Echo sounding (section)

==Technique==
{{further|Sound speed profile}}
[[File:Principle of SBES.svg|thumb|Diagram showing the basic principle of echo sounding]]
Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the [[speed of sound]] in water, which is approximately 1.5 kilometres per second. The speed of sound will vary slightly depending on temperature, pressure and salinity; and for precise applications of echosounding, such as [[hydrography]], the speed of sound must also be measured, typically by deploying a [[sound velocity probe]] in the water. Echo sounding is a special purpose application of [[sonar]] used to locate the bottom. Since a historical pre-[[International System of Units|SI]] unit of water depth was the [[fathom]], an instrument used for determining water depth is sometimes called a '''fathometer'''.

Most charted ocean depths are based on an average or standard sound speed. Where greater accuracy is required, average and even seasonal standards may be applied to ocean regions. For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to measure the temperature, pressure and salinity. These factors are used to estimate more accurately the actual sound speed in the local water column. This technique is often used by the US Office of Coast Survey for navigational surveys of US coastal waters.<ref>NOAA Field Procedures Manual, Office of Coast Survey website (http://www.nauticalcharts.noaa.gov/hsd/fpm/fpm.htm {{Webarchive|url=https://web.archive.org/web/20110810095153/http://www.nauticalcharts.noaa.gov/hsd/fpm/fpm.htm |date=10 August 2011 }})</ref>