Piloting

(navigation) The navigation of a vehicle, particularly a marine craft, by determining position relative to external reference points, usually fixed points on the earth.

The form of navigation in which position is determined relative to external reference points, usually fixed points on the Earth. It is the oldest form of navigation. With the development of electronic aids, piloting techniques were extended far from shore. However, the term “piloting” is generally associated with nearness of land, where tidal and other currents may be strong, shoals and other underwater obstructions may be in near proximity, and maneuvering room is limited when other vessels are encountered. Thus it is not unusual for ships to employ the services of a local expert, called a pilot, to assist in the navigation of the vessel while it enters or leaves port.

A conspicuous object, structure, or light that serves as an indicator for establishing the position of a craft or otherwise assisting in its safe navigation is called a mark. To be useful, not only must a mark be identified, but its position must be known accurately. Artificial marks designed and erected specifically to assist the navigator are called aids to navigation and include beacons, both lighted and unlighted, lighthouses, buoys, both lighted and unlighted, and lightships. Unlighted aids are called daymarks. See also Buoy; Lighthouse.

In addition to visible aids to navigation, bottom topography can be of assistance in locating the position of a vessel. Sound signals transmitted through water or air may be used for navigation. Electronic beacons and positioning systems have been established at a number of places to assist in navigation. See also Electronic navigation systems.

The measurements made for piloting purposes are of direction, distance, differential distance between two points, and distance to bottom.

Bearings are usually measured (1) by noting when two objects are in range (directly in line); (2) by means of a suitable attachment to a compass or compass repeater; (3) by pelorus, a compasslike instrument without directive properties; or (4) electronically, by radio direction finder, by radar, or by the indication of the receiver-indicator of an electronic system of navigation. Distance is generally measured by radar. The difference in distance from the ship to two points is usually measured electronically by means of the receiver-indicator of a hyperbolic navigation system. Depth measurement is usually made by an echo sounder. See also Direction-finding equipment; Echo sounder; Hyperbolic navigation system; Radar.

Traditionally, position by piloting has been determined by means of lines of positions, each indicating a series of possible positions of the craft at the time of measurement. A measured bearing provides a straight line of position (actually part of a great circle) passing through the object sighted. A measured distance provides a circular line of position with the object as the center and the distance as the radius. A measured differential distance provides a hyperbolic line of position. A position, called a fix, is usually determined by crossing two or more lines of position taken simultaneously or nearly so.

Digitization of a nautical chart provides the information needed for display of an electronic facsimile of a published chart. Selective determination of features to be shown makes possible the tailoring of the facsimile to individual requirements. If a radar image is added to the display, the position of the vessel relative to its surroundings is immediately apparent. A suitable electronic positioning system such as the Global Positioning System (GPS) or differential GPS (DGPS) can be used to display a symbol indicating the position of the vessel, thus providing a check on accuracy of the radar data. Other information can be added as desired. The combined display is called an electronic chart display information system (ECDIS). See also Celestial navigation; Dead reckoning; Marine navigation; Navigation; Polar navigation; Satellite navigation systems.