BATHYMETRY & BACKSCATTER
Bathymetric data represent measured or estimated depth values.
Backscatter data represent the intensity of the return of remotely-sensed data.
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Bathymetric data has been collected for centuries, primarily with the aim of improving the safety of navigation. One of the first tools used to estimate depths, the lead line, is still being used today. A lead line is basically a rope with a weight attached to the end; the weight is dropped to the bottom of the ocean and the length of the rope is measured. In some cases, the weight has a hole at the bottom that can be filled with grease or tallow that allows collecting a sediment sample. To this day, many nautical charts in use contain some of the early measurements made from lead lines. However, measurements from lead lines are not very accurate; they are impacted by variable current layers causing the line to drift, can sink into sediments, and can miss shoals and other elements dangerous for navigation. Other mechanical methods still in use in shallow waters include the sounding pole, or sounding rod. Sounding poles are less impacted by drift and can be mounted on a foot that prevents penetration into the upper sediment layers. A GPS receiver can be attached to the top of the pole to derive a more accurate position. Wire sweeps and bar sweeps are two methods that are used to ensure clearance over obstacles, once again for navigation purposes. A float wire or bar is lowered at a set depth, and the tension in the wire or the movement of the buoys indicates whether an obstacle is found at that given depth. Despite technological developments, these mechanical methods are still in use and can serve to calibrate more advanced acoustic sounding equipment and to verify measurements when necessary.
An increase in the understanding of underwater acoustics led to the development of the first echosounders in the early 1900s. Echosounders transmit sound waves in the ocean and an estimate of depth can be made by measuring the time between the transmitted and the returned echo when the speed of sound in the water is known. Most echosounders can also provide backscatter data, which correspond to the intensity of the acoustic return. Backscatter data have been found to be correlated with sediment types; hard bottoms may reflect more sound than soft bottoms that will absorb some of it. Different types of echosounders are used depending on the water depth and the need for accuracy and bottom object detection criteria (more on this soon).
Finally, optical remote sensing also enables estimating the depths of the ocean. This can be done using principles behind radar altimetry, satellite-derived bathymetry from multispectral data, or using lidar systems (more on this soon).
Collecting bathymetry data is a complex process requiring sophisticated instruments carried by moving platforms (ships, airplanes, or unoccupied systems) that must be frequently calibrated and maintained to ensure accuracy and precision. Several possible error sources exist and should be eliminated when possible, or at least accounted for, so that users have confidence in the final product. Sensor properties, the position (GPS) accuracy of the platform, movement of the platform along each directional axis, sound speed in the ocean, and the characteristics of the seabed are all factors that must be carefully evaluated and included in an error budget for the depth soundings.
Spatial resolution, quality, and uncertainty requirements will vary according to the survey area in question. The strictest requirements are typically within ports, harbors, and in shallow areas where chances are high that navigational dangers exist. In the deeper parts of the ocean, the criteria are less stringent, both because surface vessels are unlikely to encounter navigational hazards, but also because it is too time-consuming to map the deep ocean to the highest degree of accuracy and resolution. The International Hydrographic Organization has developed a standard for bathymetry mapping (IHO S-44) that is commonly used by ocean surveyors in most countries.
Once raw data has been collected, it is imported into software for processing and quality assurance (see resources below). Here, raw data are gridded to form a bathymetric surface. The Combined Uncertainty and Bathymetric Estimator (CUBE) algorithm is commonly used to weigh each individual sounding within each grid cell based on the sounding error budget, and to make an informed decision on the depth value of the grid cell. Once an operator has evaluated the surface, it is stored in an appropriate file format. Bathymetric Attributed Grid (BAG) or XYZ data are two commonly accepted file formats used to share data with end users.
How data are managed dictates how compatible they will be with other datasets. Large-scale bathymetry mapping campaigns are mostly due to government initiatives to improve the safety of navigation in national waters, however, most countries are members of the International Hydrographic Organization (IHO). The IHO has developed common standards for mapping accuracy and precision (S-44), as well as metadata standards and exchange formats (S-57) to help standardization and interoperability for international data sharing. Recently, the IHO has published an updated standard called S-100, which is more flexible than the previous version, as it allows for more data types (imagery, gridded data, 3-D products), and a time component that allows for rapidly changing data such as iceberg drift, marine weather, and ocean currents. See https://iho.int/ for more information.
In the United States, NOAA's National Ocean Service (NOS) has developed a national mapping standard, in parts using the IHO documents as minimum requirements for seabed mapping. More information is available in the resources below and will be added to this section shortly.