The RAN Hydrographic Service has responsibility for charting more than one eighth of the world's surface, stretching as far west as Cocos Island in the Indian Ocean, east to the Solomon Islands, and from the Equator to the Antarctic. The RAN has six ships and one aircraft engaged in this paramount and vital task.
The nautical charts developed from data gathered by the Hydrographic Service are essential for safe navigation at sea.
Around Australia, less than half of the area has been surveyed to acceptable standards, however the two Leeuwin Class ships, with multi-beam echo sounders, will greatly reduce this figure, making passage of vessels safer and further protect Australia's ocean environment.
|HMAS Leeuwin||A 245||Leeuwin Class||95934960027 May 2000|
|HMAS Melville||A 246||Leeuwin Class||95934960027 May 2000|
Hydrographic surveys traditionally result in the nautical chart and include all observations required to ensure safety of navigation. The fundamental difference between a nautical chart product and a map is that the user of the chart is unable to verify the detail that is hidden beneath the sea surface. The responsibility of the hydrographer is therefore to cover a survey area so that no feature remains undiscovered. This will allow the military commander maximum freedom of manoeuvre to prosecute his mission. The hydrographer as the Navy's geospatial specialist also has a warfighting role to present environmental information to the commander providing him with situational awareness.
The prime requirement of a hydrographic survey is to measure spot depths below the surface of the sea at a density that gives thorough coverage. Sounding techniques ensure that the natural relief of the seabed is adequately surveyed and all hazards to the military mission are identified. The common form of presenting this information is a chart with contoured soundings. However, modern systems provide for a variety of two and three-dimensional visualisations for the user.
Hydrographic soundings have to be collected with a consistent position framework. This framework is the horizontal control. Most modern surveys utilise satellite navigation, however, units are also equipped with the means to establish horizontal control from sites ashore.
Sounding is the operation whereby an area is methodically covered by depth measurement in order to portray the relief of the seabed. It consists of obtaining parallel profiles across the survey area. Echo sounding systems provide a depth profile of water beneath the survey vessel. Aligning a series of parallel profiles will enable the drawing of contours and provide a sense of the relief of the seabed. Side scan sonar technique utilises a towfish towed behind the survey ship, and trailed above the seabed. The transmission from the side scan sonar enables the detection of features between profiles. This system cannot determine depth accurately. Modern survey ships fitted with multibeam systems are capable of determining depth and feature detection. The multibeam system transmits a swath similar in concept to 'mowing the lawn' collecting depth information and conducting feature detection simultaneously.
Any depth measurement must be related to a common datum level. A datum may be any arbitrary level defined by its relation to a permanent mark ashore or connected to a level that can be retrieved at any time. In the Australian area of charting responsibility this datum is linked to the lowest astronomical tide (recoverable by calculation). Depths on a published chart are shown as a depth below the chart datum. As all sounding are measured by ships effected by the tide; tidal observation must be recorded at the time of sounding to correct the depth for the height of tide. Therefore tidal observation becomes a critical aspect of depth measurement; the survey ship will be required to deploy tide gauges at sea or ashore for the duration of the survey.
The texture of the seabed is valuable information in maritime warfare. Seabed texture has an influence on the performance of sonar systems and therefore has an influence in mine counter measures and anti submarine warfare. A systematic process of bottom sampling across a survey area can determine bottom texture. The result of this work is a bottom texture sheet.