Hydrography maps and describes bodies of water, including oceans, lakes, and rivers, for shipping, fishing, oil and gas exploration, research, and environmental protection. Depth is measured using acoustic techniques, and seabed composition is important for marine activities. Hydrographic services are provided by organizations such as NOAA and the UK Hydrographic Office.
Hydrography is the field of study that describes and maps the physical characteristics of bodies of water, including oceans, seas, freshwater lakes, rivers, and estuaries. It is a multidisciplinary field, including elements of oceanography, cartography, marine geology, acoustics, and other areas. The primary concern is the mapping and documentation of features that are important for shipping, commercial shipping, and fisheries, but it also addresses oil and gas exploration, defense, marine and freshwater research, and environmental protection. A hydrographic survey of an ocean or freshwater area may be undertaken in order to produce a map or chart detailing points of interest, particularly hazards to navigation such as submerged rocks, sandbars and wrecks, but also temperature, salinity and currents. Of particular importance in hydrography is the topography and nature of the seabed.
By measuring the water depth at regular and known points, a hydrographic map can be prepared. The determination of position in terms of latitude and longitude was, until the late 1970s, done visually, using sextants and triangulation methods, but electronic means and the Global Positioning System (GPS) have since been used. Until the 1930s, water depth was measured using weighted lead lines or sounding poles, after which this slow and inaccurate method was largely replaced by acoustic measurement techniques. These use a downward-sent pulse of sound from a transmitter to echo off the floor and a receiver to capture the echo. The time taken to receive the echo together with the knowledge of the speed of sound in water allows us to calculate the depth.
Single beam echo sounders (SBES) are often used in shallow water hydrography and employ a narrow single sound pulse to record the shallowest depth within the covered area, as this is of the utmost importance to navigation. Corrections have to be made to the raw data to account for tides, ship motion, and changes in the speed of sound at different depths. Multibeam echo sounders (MBES), which employ a large number of fan-shaped sound pulses, achieve more complete coverage and are typically used in deep water. These can produce very detailed maps, but are more expensive than SBES and corrections are more complicated. Sidescan sonar uses a sound source near the seabed that produces near-horizontal sound beams, providing high-resolution images of features that may not appear clearly in SBES or MBES data.
Seabed composition is important for anchoring, undersea construction, pipeline laying, and other marine activities. Samples can be obtained from cores, dredging or simply collecting material to determine its type, such as sand, mud or rock. Sometimes underwater photography is also used. This information, along with depth measurement and tidal measurement data, can be incorporated into hydrographic and nautical charts used for navigation.
Other data not related to navigation may be collected from a hydrographic survey for scientific research purposes. For example, ocean surface temperatures can be measured from space by satellites recording received infrared radiation and subsurface temperatures recorded by thermometers. Salinity can be thought of as a measure of the amount of solid material dissolved in ocean water and is generally measured by the electrical conductivity of the water. Ocean currents can be measured either by a current meter, a device that measures the strength of the current at a particular depth, or by using an Acoustic Doppler Current Profiler (ADCP), which can record currents across a range of depths.
Hydrographic services are provided by a number of organisations. In the United States, for example, there is the National Oceanic and Atmospheric Administration’s (NOAA) Office of Coast Survey, and in the United Kingdom, the UK Hydrographic Office. The International Hydrographic Organization (IHO) has set international standards for hydrographic surveys and maps.
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