Hydrographic Survey Equipment

Now that digital technology has taken over and offers positional precision formerly only dreamed of (satellite-based systems such as GPS and Glonass) even for leisure craft, the positional precision of navigational charts is coming into focus. It is thus possible that in many places outside public fairways, the precision of the navigator’s absolute position determination may be as good or better than that of the navigational chart. This may tempt the navigator to reduce the margins of safety, which may prove to be a fatal mistake.

Positioning

DGPS is normally used for positioning. Polar coordinate systems (bearing and distance) can be used in areas where GPS does not work. More sophisticated carrier wave-based GPS technology (Real Time Kinematic, RTK) is used in certain cases to obtain better horizontal position and height. In such cases height data derived from GPS can be used as an alternative to traditional tide measurement.

Depth determination

Modern hydrography poses demands for full seafloor coverage. The most comprehensive measuring technique is multibeam echo sounding. Acoustical pulses are sent out in a fan-shaped pattern resulting in a three-dimensional depiction of the entire bottom topography.

Bar-sweeping is used to check minimum depth in fairways where margins are small. Bar-sweeping is performed through mechanical sensing of the seafloor. In many cases, the checks are based on water regulations which require the Swedish Maritime Administration to verify a particular depth after dredging.

The position of fixed and floating navigational aids and other objects of nautical interest in fairways are also checked. Floating objects are normally measured from vessels. Fixed objects such as lighthouses are often measured using geodetic technology.

Other hydrographic survey equipment

As a complement to multibeam, some vessels are also equipped with “Side Scan Sonar,” which is a special multibeam echo sounder mounted on a “towfish” that is towed by the vessel as close to the bottom as possible. It is used in particular to search for small objects sticking up from the bottom. This technology makes it easier to locate wrecks.

The hydrographic service also has a smaller craft known as a “Remotely Operated Vehicle” or ROV, which can be controlled via cable from the vessel or another suitable place. The craft is equipped with cameras and grippers and can be used to more closely investigate objects found on the bottom.

Hydrographic surveys are currently related to the median water level for 2000. Reference levels for inland waters have been defined based on water regulations. Please see the relevant navigational chart for more detailed information about the reference level.