Bathymetry is the study of underwater depth of lake or ocean floors. In other
words, bathymetry is the underwater equivalent to hypsometry or topography.
The name comes from Greek βαθύς, "deep", and μέτρον, "measure". 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.
Paleobathymetry is the study of past underwater depths.
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 only a singular point at a time, and is therefore 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 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.
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 boat-mounted Global Positioning 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. As of 2010 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 ca. 2003-2005,
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.
See also Bathymetric chart
General Bathymetric Chart of the Oceans Bathometer
Terrain Hydrographic survey
Ocean surface topography References
External links Overview for underwater terrain, data
formats, etc. High resolution bathymetry for the Great
Barrier Reef and Coral Sea A.PO.MA.B.-Academy of Positioning Marine
and Bathymetry