Navigation

The predicted yearly rate of change in magnetic variation at a stated place and epoch. It is printed on charts or other magnetic information so an older variation can be adjusted approximately to the required year. Because the rate itself can change, current chart information should be used for important navigation.

The meridian of longitude approximately 180 degrees from a chosen prime meridian. The antimeridian to Greenwich lies at 180° east or west and broadly coincides with parts of the International Date Line. Crossing the date line changes the calendar date, although political deviations mean it does not follow the meridian exactly.

The initial passage-planning stage in which all relevant information is gathered and assessed. Charts, publications, forecasts, tides, vessel limitations, regulatory requirements and available navigational aids are reviewed before the route is planned. Missing, unreliable or conflicting information should be resolved or explicitly allowed for.

The horizontal direction of an object measured clockwise from true north. It is expressed from 000° to 359° and must be labelled as true to avoid confusion with magnetic, compass or relative bearings. True bearings can be plotted directly on a correctly oriented chart.

Sufficient unobstructed water space for a vessel to manoeuvre safely. The amount required depends on draught, speed, turning ability, stopping distance, weather and proximity to shoals, traffic and a lee shore. Maintaining sea room preserves options if conditions deteriorate or equipment fails.

The direction and rate at which current carries a vessel over the ground. Set is the direction toward which the water flows and drift is its speed. Together they form a current vector used with the vessel’s through-water motion to predict track or calculate a course to steer.

A mathematical reference framework used to define positions on or near the Earth. A datum specifies an ellipsoid and its relationship to the Earth, allowing latitude, longitude and height coordinates to be interpreted consistently. Positions based on different datums can be displaced, so chart, receiver and exchanged coordinates must use compatible references.

An approximate relative bearing expressed by imagining the vessel or observer at the centre of a clock face. Twelve o’clock is directly ahead, three o’clock to starboard, six o’clock astern and nine o’clock to port. It is quick for lookout reports but less precise than a bearing in degrees.