hydrography

hydrography, the science of marine surveying and of determining the position of points and objects on the globe's surface, depths of the sea, etc.

Most of the world's hydrographic knowledge and practice has come from the operations of the world's navies. A general requirement, before the birth of the various national hydrographic departments, was for the masters, or navigators, of the naval ships of all nations to observe, survey, and report to their respective admiralties on all possible occasions. Towards the end of the 18th century most maritime nations had thus collected large volumes of mainly uncollated hydrographic information, and most of them appointed official hydrographers during the course of the next half-century to organize the mass of knowledge and to publish it for the benefit of seamen of all nations.

The extent of a nation's overseas possessions or trade initially governed each maritime country's source of information and its ability as well as its need to produce charts for its own naval and merchant shipping. France soon built up, and has since maintained, an extensive coverage and was the first to establish a hydrographer's department, in 1720. She was followed by Denmark (1784), Britain (1795), Spain (1800), the US Coast and Geodetic Survey (1816), Russia (1827), Germany (1861), Japan (1871), Italy and Sweden (1872), Norway, the Netherlands, and Chile (1874). Others followed and in 1921 the International Hydrographic Bureau at Monaco was formed by interested nations, the objective being to promote rapid and informed exchange of hydrographic information in standardized terms between member states. Information is exchanged by the United Nations-sponsored International Hydrographic Organization which was formed in 1970.

Maritime countries have commonly organized their surveying services within their navies, since the information obtained was often of national significance whether for military or commercial purposes. However, notable exceptions to this general rule include the French Corps of Hydrographic Engineers and the US Coast and Geodetic Survey, now an office within the National Ocean Service, a part of the National Oceanic and Atmospheric Administration, which has responsibilities for US home waters. The development of the methods used in hydrography from the 18th century onwards can be found in Alexander Dalrymple's Essay on Nautical Surveying (1771)
, Murdoch Mackenzie's Treatise on Maritime Surveying (1774)
, Edward Belcher's Treatise on Nautical Surveying (1835)
, William Wharton's Hydrographical Surveying (1882), U.S. Coast and Geodetic Survey (Washington, 1963), Admiralty Manual of Hydrographic Surveying (1968)
, G. S. RitchieM , The Admiralty Chart (1967)
, and A. Day , The Admiralty Hydrographic Service 1793–1919 (1967).

Hydrography Today by Adam Kerr

Originally, hydrography had a very broad remit of studying the sea and all that it contained, but it is now mainly concentrated on seafloor surveys and studies of tidal phenomena. Until the middle of the 20th century the results of this work were primarily used for chartmaking. However, the development of interest in offshore petroleum deposits led to the need for hydrography on a broader commercial base for such tasks as positioning offshore oil and gas rigs and laying pipelines on the seafloor. There is a rather indistinct line between oceanography and hydrography because the former is involved in a very broad range of maritime studies, particularly the tidal phenomena.

For over 200 years the technology of hydrography changed very little, with survey ships being positioned by celestial navigation and the depths measured by lead line. However in the 1920s it was found possible to measure depths by sending out an acoustic impulse and measuring the time for it to travel to the seafloor, be reflected, and return to the instrument. Another major development was the result of the needs of the Second World War (1939–45) to position aircraft and ships by hyperbolic navigation that allowed survey ships to be positioned at all times and in all weather conditions. The final step in providing precise positioning of survey vessels, and consequently the depths and other data that were collected, was the introduction of satellite navigation. Initially this was based on the Doppler (changing frequency) of satellite signals, as the satellite passed along polar orbits. In the 1970s GPS (global positioning system) and its Russian counterpart, Glonass, were introduced. These systems are very precise indeed, not only in the horizontal plane but also in the vertical height determination.

Depth measurement has also undergone great improvement. The early acoustic echo sounders measured only a single profile of depth under the ship as it moved along. This was much better than the single-point lead-line measurements, but still left much to be interpreted between the sounding lines, which were normally arranged as a series of parallel tracks. To ensure that no obstructions extended from the seabed between the measured depth profiles side-scan sonar was introduced. This device sent out acoustic signals perpendicular to the ship's track and, although it did not provide a precise measurement of the depths, the strength of the reflected signals from the seafloor gave an indication of changing depths or changing type of material on the bottom.

The most recent development in acoustic technology has been multi-beam echo sounders. This is similar to side-scan sonar except that it provides a fan of acoustic beams, again perpendicular to the ship's track, but now able to provide precise measurements of the depth along a swathe of the seafloor. When the survey plan is arranged with sufficiently close parallel swathes it is possible to obtain practically total measurement of the entire seafloor, leaving little to be interpolated or interpreted between measured points. The technology is particularly useful in deep water where the swathe can be very wide and large areas of the seafloor can be rapidly measured.

In very clear water a system using lasers from an aircraft, called LIDAR (Light Detection and Ranging), is now available that can be used to measure depths to approximately two to three times the visible depth, which in extremely clear water may be as much as 60 metres (197 ft). The system uses a red and a green laser that are fired at the water beneath the aircraft. Because of its higher frequency, the red laser is reflected from the water surface, but the green laser penetrates to the bottom, and the difference of return time of these two signals provides a measure of the water depth. The system has been used to a great extent in complex areas such as the Great Barrier Reef.

These new depth-measuring systems collect vast amounts of data, and processing it has become a major challenge. All survey ships now carry computer systems that process it on line, and in some cases produce digital plots on board. Data visualization has become an important task both for the purpose of refining survey work while still in the field, and as a final product in forms such as the electronic chart for navigators and other users.