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Bathymetry is the underwater equivalent to hypsometry. The name comes from Greek ßa???, deep, and µet???, measure. In other words, bathymetry is the study of underwater depth, of the third dimension of lake or ocean floors. A bathymetric map or chart usually shows floor relief or terrain as contour lines (called depth contours or isobaths), and may additionally provide surface navigational information. Originally, bathymetry referred to the measurement of ocean depth. Early techniques used pre-measured heavy rope or cable lowered over a ship's side. The greatest limitation of this technique is that it measures the depth only a single point at a time, and so is inefficient. It is also subject to movements of the ship and currents moving the line out of true and therefore is inaccurate. 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 or LADAR systems [1]. 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 tells the equipment how far down the seafloor is. LIDAR/LADAR surveys are usually conducted by airborne systems. Years ago, the occasional pings of a single-beam sounder might be averaged to make a map. Today, a multibeam echosounder may be used, featuring dozens of very narrow adjacent beams arranged in a fan-like swath of perhaps 90 to 180 degrees across. The tightly packed array of narrow individual beams provides very high angular resolution and accuracy. In general, the wide swath, which is depth dependent, allows a boat to map more seafloor in less time by making fewer passes. The beams update many times per second (typically 1-40 Hz depending on water depth), allowing faster boat speed while maintaining 100% coverage of the seafloor. Attitude sensors correct for the boat's roll, pitch and yaw on the ocean surface, and a gyrocompass provides accurate heading information to correct for vessel yaw. The Global Positioning System specifies where the boat is. Sound velocity profiles (speed of sound in water) 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. In the end, a map is semi-automatically generated from this massive trove of data. 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 abyssal plains and trenches[2].
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