Editing Bathymetry (section)

==Measurement==
{{see also|Hydrographic survey#Methods}}

[[File:Rear map.jpg|thumb|250px|First printed map of oceanic bathymetry, published by [[Matthew Fontaine Maury]] with data from [[USS Dolphin (1836)|USS ''Dolphin'']] (1853)]]

Originally, bathymetry involved the measurement of [[ocean]] depth through [[depth sounding]]. Early techniques used pre-measured heavy [[rope]] or cable lowered over a ship's side.<ref name="NGA">{{cite web |first=Furlong |last=Audrey |url=https://www.youtube.com/watch?v=gtKMrMy6arw |publisher=[[National Geospatial-Intelligence Agency]] via [[YouTube]] |title=NGA Explains: What is hydrography? |date=November 7, 2018}}</ref> This technique measures the depth at one point at a time, and is therefore less efficient than other methods. It is also subject to movements of the ship and currents moving the line out of true, and thus is also less accurate.

The data used to make bathymetric maps today typically comes from an echosounder ([[sonar]]) mounted beneath or over the side of a boat, "pinging" a beam of sound downward at the seafloor or from [[remote sensing]] [[Lidar|LIDAR]] or LADAR systems.<ref name='Olsen'>{{citation  | last = Olsen  | first = R. C.  | author-link =
  | title = Remote Sensing from Air and Space  | publisher = SPIE  | year = 2007  | isbn = 978-0-8194-6235-0| url = https://www.spiedigitallibrary.org/samples/PM162.pdf  }}</ref> The amount of time it takes for the sound or light to travel through the water, bounce off the seafloor, and return to the sounder informs the equipment of the distance to the seafloor. LIDAR/LADAR surveys are usually conducted by airborne systems.

[[File:Atlantic-trench.JPG|thumb|250px|The seafloor [[topography]] near the [[Puerto Rico Trench]]]]
[[File:AYool topography 15min.png|thumb|250px|right|Present-day [[Earth]] bathymetry (and [[terrain|altimetry]]).  Data from the [[National Centers for Environmental Information]]'s [http://www.ngdc.noaa.gov/mgg/topo/ TerrainBase Digital Terrain Model].]]

Starting in the early 1930s, single-beam sounders were used to make bathymetry maps. Today, [[multibeam echosounder]]s (MBES) are typically used, which use hundreds of very narrow adjacent beams (typically 256) arranged in a fan-like [[swath width|swath]] of typically 90 to 170 degrees across. The tightly packed array of narrow individual beams provides very high [[angular resolution]] and accuracy. In general, a wide swath, which is depth dependent, allows a boat to map more seafloor in less time than a single-beam echosounder by making fewer passes. The beams update many times per second (typically 0.1–50 [[Hertz|Hz]] depending on water depth), allowing faster boat speed while maintaining 100% coverage of the seafloor.  Attitude sensors allow for the correction of the boat's [[flight dynamics|roll and pitch]] on the ocean surface, and a [[gyrocompass]] provides accurate heading information to correct for vessel [[flight dynamics|yaw]].  (Most modern MBES systems use an integrated motion-sensor and position system that measures yaw as well as the other dynamics and position.) A satellite-based global navigation system positions the soundings with respect to the surface of the earth. Sound speed profiles (speed of sound in water as a function of depth) of the water column correct for [[refraction]] or "ray-bending" of the sound waves owing to non-uniform water column characteristics such as temperature, [[Conductivity (electrolytic)|conductivity]], and pressure. A computer system processes all the data, correcting for all of the above factors as well as for the angle of each individual beam.  The resulting sounding measurements are then processed either manually, semi-automatically or automatically (in limited circumstances) to produce a map of the area.  {{As of | 2010}} a number of different outputs are generated, including a sub-set of the original measurements that satisfy some conditions (e.g., most representative likely soundings, shallowest in a region, etc.) or integrated [[Digital Terrain Models|digital terrain models]] (DTM) (e.g., a regular or irregular grid of points connected into a surface). Historically, selection of measurements was more common in [[Hydrography|hydrographic]] applications while DTM construction was used for engineering surveys, geology, flow modeling, etc.  Since {{Circa|2003}}–2005, DTMs have become more accepted in hydrographic practice.

[[Satellite]]s are also used to measure bathymetry. Satellite radar maps deep-sea topography by detecting the subtle variations in sea level caused by the gravitational pull of [[seamount|undersea mountains]], [[Mid-ocean ridge|ridges]], and other masses. On average, sea level is higher over mountains and ridges than over [[abyssal plain]]s and [[oceanic trench|trenches]].<ref name="Thurman">{{citation
| last = Thurman
| first = H. V.
| year = 1997
| title = Introductory Oceanography
| publisher = Prentice Hall College
| location = New Jersey, USA
| isbn = 0-13-262072-3
}}</ref>

In the [[United States]] the [[United States Army Corps of Engineers]] performs or commissions most surveys of navigable inland waterways, while the [[National Oceanic and Atmospheric Administration]] (NOAA) performs the same role for ocean waterways.  Coastal bathymetry data is available from [[National Oceanic and Atmospheric Administration|NOAA's]] [[National Geophysical Data Center]] (NGDC),<ref>{{cite web|url=https://www.ncei.noaa.gov/products/seafloor-mapping |title=Bathymetry and Global Relief |website=www.ngdc.noaa.gov |publisher=NOAA National Centers for Environmental Information |access-date=8 July 2022 }}</ref> which is now merged into [[National Centers for Environmental Information]].  Bathymetric data is usually referenced to tidal vertical [[datum (geodesy)|datum]]s.<ref>{{cite web|url=https://www.ncei.noaa.gov/products/coastal-elevation-models |title=Coastal Elevation Models |website=www.ngdc.noaa.gov |date=15 September 2020 |publisher=NOAA National Centers for Environmental Information |access-date=8 July 2022 }}</ref>  For deep-water bathymetry, this is typically Mean Sea Level (MSL), but most data used for nautical charting is referenced to Mean Lower Low Water (MLLW) in American surveys, and Lowest Astronomical Tide (LAT) in other countries.  Many other [[datum (geodesy)|datum]]s are used in practice, depending on the locality and tidal regime.

Occupations or careers related to bathymetry include the study of oceans and rocks and minerals on the ocean floor, and the study of underwater [[submarine earthquake|earthquakes]] or [[submarine volcano|volcanoes]].  The taking and analysis of bathymetric measurements is one of the core areas of modern [[hydrography]], and a fundamental component in ensuring the safe transport of goods worldwide.<ref name="NGA"/>

[[File:Earth_dry_elevation.stl|thumb|[[STL (file format)|STL 3D model]] of Earth without liquid water with 20× elevation exaggeration]]

===Satellite imagery===
{{further|Satellite imagery|Satellite-derived bathymetry}}

Another form of mapping the seafloor is through the use of satellites. The satellites are equipped with [[hyperspectral imaging|hyper-spectral]] and [[multispectral imaging|multi-spectral]] sensors which are used to provide constant streams of images of coastal areas providing a more feasible method of visualising the bottom of the seabed.<ref name=F2>Charles W. Finkl, ed., 2016, ''Seafloor Mapping Along Continental Shelves: Research and Techniques for Visualizing Benthic Environments.'' Internet resource edition. Volume 13. pp. 31–35</ref>

====Hyper-spectral sensors====
{{main|Hyperspectral imaging}}
The data-sets produced by hyper-spectral (HS) sensors tend to range between 100 and 200 [[spectral band]]s of approximately 5–10&nbsp;nm bandwidths. Hyper-spectral sensing, or imaging spectroscopy, is a combination of continuous remote imaging and spectroscopy producing a single set of data.<ref name=F2/> Two examples of this kind of sensing are AVIRIS ([[airborne visible/infrared imaging spectrometer]]) and HYPERION.

The application of HS sensors in regards to the imaging of the seafloor is the detection and monitoring of [[chlorophyll]], [[phytoplankton]], [[salinity]], water quality, dissolved organic materials, and [[suspended load|suspended sediments]]. However, this does not provide a great visual interpretation of coastal environments.<ref name=F2/>{{clarify|What relevance does this technology have to actual seafloor mapping?|date=July 2022}}

====Multi-spectral sensors====
{{main|Multispectral imaging}}
The other method of satellite imaging, multi-spectral (MS) imaging, tends to divide the EM spectrum into a small number of bands, unlike its partner hyper-spectral sensors which can capture a much larger number of spectral bands.

MS sensing is used more in the mapping of the seabed due to its fewer spectral bands with relatively larger bandwidths. The larger bandwidths allow for a larger spectral coverage, which is crucial in the visual detection of marine features and general spectral resolution of the images acquired.<ref name=F2/>{{clarify|How is it used? How is the larger spectral coverage relevant to visual detection of features?|date=July 2022}}

===Airborne laser bathymetry===
{{main|Airborne lidar bathymetry}}

High-density airborne laser bathymetry (ALB) is a modern, highly technical,{{citation needed|date=February 2025}} approach to the mapping the seafloor. First developed in the 1960s and 1970s,{{citation needed|date=July 2022}} ALB is a "light detection and ranging (LiDAR) technique that uses [[visible spectrum|visible]], [[ultraviolet]], and near [[infrared]] light to optically remote sense a contour target through both an active and passive system." This means that airborne laser bathymetry also uses light outside the visible spectrum to detect curves in the underwater landscape.<ref name=F2/>

[[LiDAR]] (Light Detection and Ranging) is, according to the [[National Oceanic and Atmospheric Administration]],  "a remote sensing method that uses light in the form of a [[pulsed laser]] to measure distances".<ref name=noaa/> These light pulses, along with other data, generate a [[three-dimensional]] representation of whatever the light pulses reflect off, giving an accurate representation of the surface characteristics. A LiDAR system usually consists of a [[laser]], scanner, and [[GPS]] receiver. Airplanes and helicopters are the most commonly used platforms for acquiring LIDAR data over broad areas. One application of LiDAR is bathymetric LiDAR, which uses water-penetrating green light to also measure seafloor and riverbed elevations.<ref name=noaa>{{cite web |author=National Oceanic and Atmospheric Administration (NOAA)| title=What is LIDAR? | publisher=National Ocean Service | date=15 April 2020 | url=https://oceanservice.noaa.gov/facts/lidar.html | access-date=21 June 2020}}</ref>

ALB generally operates in the form of a pulse of non-visible light being emitted from a low-flying aircraft and a receiver recording two reflections from the water. The first of which originates from the surface of the water, and the second from the seabed. This method has been used in a number of studies to map segments of the seafloor of various coastal areas.<ref>Brock & Purkis (2009). "The emerging role of Lidar remote sensing in coastal research and resource management". In: Brock J, Purkis S (eds.). "Coastal applications of airborne lidar". ''Journal of Coastal Research'', Special Issue No. 53: pp. 1–5</ref><ref>Bukata et al. (1995) ''Optical properties and remote sensing of inland and coastal waters.'' CRC Press, Boca Raton, p. 365</ref><ref>Deronde et al. (2008). "Monitoring of the sediment dynamics along a sandy shoreline by means of airborne hyper-spectral remote sensing and LIDAR: a case study in Belgium". ''Earth Surface Processes'' 33: pp. 280–294</ref>

====Examples of commercial LIDAR bathymetry systems====
There are various LIDAR bathymetry systems that are commercially accessible. Two of these systems are the Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) and the Laser Airborne Depth Sounder (LADS). SHOALS was first developed to help the [[United States Army Corps of Engineers]] in bathymetric surveying by a company called Optech in the 1990s. SHOALS is done through the transmission of a laser, of wavelength between 530 and 532&nbsp;nm, from a height of approximately 200&nbsp;m at speed of 60&nbsp;m/s on average.<ref>Charles W. Finkl, ed., 2016, ''Seafloor Mapping Along Continental Shelves: Research and Techniques for Visualizing Benthic Environments.'' Internet resource edition. Volume 13. p. 23</ref>

===High resolution orthoimagery===
{{Further|Orthophoto}}
High resolution orthoimagery (HRO) is the process of creating an image that combines the geometric qualities with the characteristics of photographs. The result of this process is an [[orthophoto|orthoimage]], a scale image which includes corrections made for feature displacement such as building tilt. These corrections are made through the use of a mathematical equation, information on sensor calibration, and the application of digital elevation models.<ref name=r7>USGS, Date Last Edited 2015, ''High Resolution Orthoimagery (HRO)'', https://lta.cr.usgs.gov/high_res_ortho</ref>

An orthoimage can be created through the combination of a number of photos of the same target. The target is photographed from a number of different angles to allow for the perception of the true elevation and tilting of the object. This gives the viewer an accurate perception of the target area.<ref name=r7/>

High resolution orthoimagery is currently being used in the 'terrestrial mapping program', the aim of which is to 'produce high resolution topography data from Oregon to Mexico'. The orthoimagery will be used to provide the photographic data for these regions.<ref>State of California Ocean Protection Council, 2009, ''Mapping California's Resources'', http://www.opc.ca.gov/2009/12/mapping/</ref>