Bathymetry
Bathymetry is the study of underwater depth of ocean floors, river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water depth measurements are from Ancient Egypt over 3000 years ago. Bathymetry has various uses including the production of bathymetric charts to guide vessels and identify underwater hazards, the study of marine life near the floor of water bodies, coastline analysis and ocean dynamics, including predicting currents and tides.
Bathymetric charts, are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines and selected depths, and typically also provide surface navigational information. Bathymetric maps may also use a digital terrain model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a global relief model. Paleobathymetry is the study of past underwater depths.
Synonyms include seafloor mapping, seabed mapping, seafloor imaging and seabed imaging. Bathymetric measurements are conducted with various methods, from depth sounding, sonar and lidar techniques, to buoys and satellite altimetry. Various methods have advantages and disadvantages and the specific method used depends upon the scale of the area under study, financial means, desired measurement accuracy, and additional variables. Despite modern computer-based research, the ocean seabed in many locations is less measured than the topography of Mars.
Seabed topography
Measurement
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. 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 mounted beneath or over the side of a boat, "pinging" a beam of sound downward at the seafloor or from remote sensing LIDAR or LADAR systems. 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. Data from the National Centers for Environmental Information's .
Starting in the early 1930s, single-beam sounders were used to make bathymetry maps. Today, multibeam echosounders are typically used, which use hundreds of very narrow adjacent beams arranged in a fan-like 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, allowing faster boat speed while maintaining 100% coverage of the seafloor. Attitude sensors allow for the correction of the boat's roll and pitch on the ocean surface, and a gyrocompass provides accurate heading information to correct for vessel yaw. A satellite-based global navigation system positions the soundings with respect to the surface of the earth. Sound speed profiles 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, 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 to produce a map of the area. a number of different outputs are generated, including a sub-set of the original measurements that satisfy some conditions or integrated digital terrain models . Historically, selection of measurements was more common in hydrographic applications while DTM construction was used for engineering surveys, geology, flow modeling, etc. Since –2005, DTMs have become more accepted in hydrographic practice.
Satellites 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 undersea mountains, ridges, and other masses. On average, sea level is higher over mountains and ridges than over abyssal plains and trenches.
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 performs the same role for ocean waterways. Coastal bathymetry data is available from NOAA's National Geophysical Data Center, which is now merged into National Centers for Environmental Information. Bathymetric data is usually referenced to tidal vertical datums. For deep-water bathymetry, this is typically Mean Sea Level, but most data used for nautical charting is referenced to Mean Lower Low Water in American surveys, and Lowest Astronomical Tide in other countries. Many other datums 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 earthquakes or 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.
Satellite imagery
Another form of mapping the seafloor is through the use of satellites. The satellites are equipped with hyper-spectral and 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.Hyper-spectral sensors
The data-sets produced by hyper-spectral sensors tend to range between 100 and 200 spectral bands of approximately 5–10 nm bandwidths. Hyper-spectral sensing, or imaging spectroscopy, is a combination of continuous remote imaging and spectroscopy producing a single set of data. Two examples of this kind of sensing are AVIRIS 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 sediments. However, this does not provide a great visual interpretation of coastal environments.
Multi-spectral sensors
The other method of satellite imaging, multi-spectral 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.
Airborne laser bathymetry
High-density airborne laser bathymetry is a modern, highly technical, approach to the mapping the seafloor. First developed in the 1960s and 1970s, ALB is a "light detection and ranging technique that uses 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.LiDAR 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". 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.
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.
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 and the Laser Airborne Depth Sounder. 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 nm, from a height of approximately 200 m at speed of 60 m/s on average.High resolution orthoimagery
High resolution orthoimagery is the process of creating an image that combines the geometric qualities with the characteristics of photographs. The result of this process is an 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.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.
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.