Navigation

The straight-line distance between two positions actually reached by a vessel. It measures net progress over the ground rather than the total distance travelled along every alteration of course. Together with elapsed time it can be used to calculate speed made good.

A buoy that is no longer held in its charted or intended position and is moving with wind or current. It may give dangerously misleading navigational information and should not be relied upon as a fixed mark. Its position and description should be reported promptly to the appropriate authority.

A controlled series of heading observations used to determine a magnetic compass’s deviation. The vessel is placed on known headings or compared with reliable references around the compass. Results are recorded as a deviation table or card and repeated after changes likely to affect the vessel’s magnetic field.

The scale at which source data was selected and compiled for a chart or electronic navigational dataset. It indicates the intended level of detail and use but does not guarantee positional accuracy. Displaying electronic chart data at a larger scale cannot create detail or precision absent from the source.

The convention used to write a geographic or grid position. Latitude and longitude may be expressed as degrees and decimal minutes, decimal degrees, or degrees, minutes and seconds. The format, hemispheres and datum must be stated because misreading them can place a position many miles away.

The practical use of nautical charts to plan, plot and monitor a vessel’s navigation. It includes positions, courses, distances, bearings, tidal vectors, clearing limits and other constructions. Accurate chartwork depends on correct symbols, datums, scales and time references and must be checked against observations and electronic systems.

The direction of an object measured clockwise from compass north. It includes both magnetic variation and the observing compass’s deviation. Applying deviation gives a magnetic bearing and then applying variation gives a true bearing, using a consistent sign convention.

The graduated directional display in a magnetic compass. It is marked in degrees and often cardinal and intercardinal points and aligns with the local magnetic field. The vessel’s heading is read at the lubber line while allowance is made for deviation, heel and reading error.

The angular difference between the direction indicated by a magnetic compass and true direction. It is the combined effect of magnetic variation and compass deviation, with sign determined by the adopted convention. Error can be found by comparing compass observations with a reliable true bearing or transit.

A line of constant longitude shown on a nautical chart. Meridians run generally north–south and, with parallels of latitude, form the chart’s coordinate graticule. Their appearance and spacing depend on the chart projection, while their labels provide the longitude scale.