Bathymetry is the underwater equivalent to altimetry. The name comes from Greek βαθυς, deep, and μετρον, measure. A bathymetric map gives the depth contours of the soil, rock, sand, etc. at the bottom of a body of water such as an ocean or a lake. In addition to this, a bathymetric chart provides navigational information.

The data used to make bathymetric maps typically comes from an echosounder (sonar) mounted beneath or over the side of a boat, 'pinging' a "beam," or sound, downward at the seafloor. The amount of time it takes for the sound to make it through the water, bounce off the seafloor and come back up to the sounder, tells the equipment how far down the seafloor is.

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 resolution detail and accuracy. In general, the wide swath, which is depth dependent, allows the 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. Motion 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. Global positioning systems specify exactly where the boat is. Accurate 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.

Most surveys of navigable waterways in the US are performed or commissioned by the United States Army Corps of Engineers, for inland waterways, and the National Oceanic and Atmospheric Administration for oceans.

See also: topography

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Geomorphology | Cartography

Bathymetrie | Bathymétrie | Batimetria | Batymetri | Batimetria