Extended Operational and Trade Vocabulary

Nautical Science, Navigation and COLREGs glossary

The formal navigation and collision-avoidance vocabulary: position accuracy and fix terminology, ARPA target acquisition, abnormal refraction, celestial and terrestrial methods, and the COLREGs rules of the road. Grounds each term in the navigational practice and the collision regulation it serves, the precise counterpart to the deck-seamanship section.

734 defined terms.

Showing 250 on this page (page 1 of 3).

A

Abeam: Bearing 090° relative to ship's heading.
Abnormal refraction: Atmospheric condition causing celestial or terrestrial objects to appear displaced from their true position.
Absolute humidity: Mass of water vapor per unit volume of air.
Accuracy: Degree of conformance between an estimated or measured position and the true position.
Acquisition (ARPA): Process of selecting a radar target for automatic tracking.
Adiabatic Lapse Rate: Rate at which an air parcel cools or warms as it rises or descends without exchanging heat with surroundings.
Admiralty Chart: Nautical chart issued by the UK Hydrographic Office.
Admiralty Information Overlay (AIO): ENC overlay supplied by UKHO showing Temporary and Preliminary Notices and ENC Preliminary Notices.
Admiralty List of Lights and Fog Signals (ALL): UKHO publications NP74 to NP84, volumes A through L, listing lighted aids worldwide.
Admiralty List of Radio Signals (ALRS): UKHO series NP281 to NP286 covering radio communications, satellite services, radio aids, meteorology, GMDSS and pilot services.
Admiralty Notices to Mariners (NTM): Weekly UKHO bulletin promulgating chart and publication corrections.
Admiralty Sailing Directions: UKHO Pilots, NP1 to NP72, giving coastal descriptions and navigational information.
Admiralty Tide Tables (ATT): UKHO publications NP201 to NP204 giving tidal predictions worldwide.
Advection fog: Fog formed when warm moist air moves over a colder surface.
Age of tide: Time lag between syzygy and the following spring tide.
Agonic line: Line on the Earth's surface along which magnetic variation is zero.
Aircraft Reply (CPA-related): Term used loosely; see CPA.
AIS (Automatic Identification System): Class A/B transponder per ITU-R M.1371.
AIS Class A: Mandatory transponder for SOLAS vessels broadcasting MMSI, position, course, speed.
AIS Class B: Lower-power transponder used by smaller, non-SOLAS craft.
AIS-SART: AIS Search and Rescue Transmitter for survival craft.
Aldis lamp: Hand-held signal lamp for Morse signaling between ships.
Allision: contact between a moving vessel and a fixed object such as a wharf or bridge.
Almanac (Nautical): Annual ephemeris tables for celestial navigation.
Almucantar: Small circle on the celestial sphere parallel to the horizon; circle of equal altitude.
Alphanumeric: Display showing letters and digits, used for ECDIS and ARPA data readouts.
Altitude (celestial): Angle between horizon and celestial body.
Altitude corrections: Adjustments to sextant altitude for index error, dip, refraction, semidiameter and parallax.
Altitude observed (Ho): Sextant altitude corrected for all errors; used in sight reduction.
Altitude sextant (Hs): Angle read directly from the sextant micrometer and arc.
Ambient light: Background illumination affecting bridge night-vision and ECDIS display palettes.
Amplitude: Angle between the prime vertical and a body on the horizon; used to obtain compass error at rising or setting.
Anabatic wind: Upslope wind caused by daytime heating.
Anchor watch: Watch maintained on anchored vessel.
Aneroid barometer: Barometer using a partial vacuum cell rather than mercury.
Annex I (COLREGs): Positioning and technical details of lights and shapes.
Annex II (COLREGs): Additional signals for fishing vessels fishing in close proximity.
Annex III (COLREGs): Technical details of sound signal appliances.
Annex IV (COLREGs): Distress signals.
Anomalistic month: Time between successive perigees of the Moon.
Antenna height correction: Adjustment for height of GNSS or radar antenna above the waterline.
Anticyclone: High-pressure system with clockwise circulation in the Northern Hemisphere.
Aphelion: Point in Earth's orbit farthest from the Sun.
Apogee: Point in the Moon's orbit farthest from Earth.
Apparent motion: Motion of one vessel as seen from another moving vessel.
Apparent time: Time based on the actual position of the Sun.
Apparent wind: Wind perceived from a moving vessel.
Appraisal stage: First stage of passage planning under IMO Resolution A.893(21).
Arc of visibility (light): Angular sector through which a navigation light is visible.
ARPA (Automatic Radar Plotting Aid): Auto tracking and CPA/TCPA per IMO Res. A.823(19).
Assumed latitude: Latitude chosen near the DR for tabular sight reduction.
Assumed longitude: Longitude chosen so LHA is a whole degree for sight reduction.
Astronomical refraction: Bending of light through Earth's atmosphere when observing celestial bodies.
Astronomical triangle: Celestial PZX triangle for sight reduction.
Astronomical twilight: Period when the Sun is between 12 degrees and 18 degrees below the horizon.
Atlantic Pilot (Admiralty): Sailing direction volumes covering the Atlantic Ocean.
Augmentation: Apparent increase in the Moon's semidiameter when near the zenith.
Aurora: Polar atmospheric phenomenon caused by charged particles.
Autopilot: Automatic steering system using gyro or magnetic compass input.
Awash: With the surface just covered by water.
Azimuth: Bearing of a celestial body measured from true north.
Azimuth circle: Ring fitted over a compass repeater for taking bearings.

B

Back bearing: Reciprocal of a forward bearing, used to check compass error.
Backing (wind): Counterclockwise change in wind direction in the Northern Hemisphere.
Baltic Pilot: UKHO Sailing Directions for the Baltic Sea.
Bar (pressure): Unit of atmospheric pressure equal to 100,000 pascals; 1 millibar equals 1 hectopascal.
Barograph: Self-recording aneroid barometer.
Barometric tendency: Change in atmospheric pressure over the previous three hours.
Beam bearing: Bearing 90 degrees from the vessel's heading.
Beam wind: Wind blowing at right angles to the vessel's course.
Bearing: Direction to an object from the observer.
Beaufort wind force scale: Empirical scale from 0 (calm) to 12 (hurricane) relating wind speed to sea conditions.
Bench mark: Permanent mark of known elevation used as a tidal datum reference.
Berthing manual: Vessel-specific document detailing safe berthing procedures.
Binnacle: Stand housing the steering compass.
Bowditch: The American Practical Navigator, NIMA/NGA Publication 9.
Box the compass: Recite the thirty-two points in order.
BPG (Bridge Procedures Guide): ICS publication setting out bridge watchkeeping procedures and checklists.
Bridge management: Application of resource management principles to bridge operations (BRM).
Bridge team: Personnel comprising the navigational watch.
Bridge wing: Outdoor extension of the bridge providing visibility along the ship's side.
British Admiralty (BA): Common reference to UKHO products.
Buoy (lateral): Aid marking the side of a channel under IALA Region A or B.
Buoyage (IALA): Maritime buoyage system A (red to port entering) and B (red to starboard entering).
Buys Ballot's law: With back to the wind in the Northern Hemisphere, low pressure is to the left.

C

Cardinal mark: IALA buoy indicating safe water lies to the named cardinal side.
Catenary: sag of a mooring line or shore-power cable between supports.
Celestial equator: Projection of Earth's equator onto the celestial sphere.
Celestial fix: Position from intercept of two or more celestial LOPs.
Celestial horizon: Great circle 90 degrees from the observer's zenith.
Celestial meridian: Great circle passing through the celestial poles and the observer's zenith.
Celestial navigation: Position determination by observing altitudes of celestial bodies.
Celestial pole: Projection of Earth's pole onto the celestial sphere.
Celestial sphere: Imaginary sphere with stars projected onto it.
Centi-bar: Unit equal to 10 millibars; rarely used at sea.
Chart catalogue: NP131 UKHO publication listing all Admiralty charts and publications.
Chart correction: Manual or T&P amendment to a paper or ENC chart from NTM.
Chart datum (CD): Reference for sounding depths, usually LAT.
Chart projection: Method of representing the curved Earth on a flat surface.
Chart symbols: Standardized symbols defined in NP5011 (UKHO) and Chart No. 1 (NOAA).
Chronometer: Precision timekeeper carrying UT1/UTC for celestial navigation.
Circular error probable (CEP): Radius within which 50 percent of position fixes fall.
Clearing bearing: Bearing line used to clear a known danger.
Clearing mark: Visible mark used to keep a vessel clear of a danger.
Closest point of approach (CPA): Minimum range between own ship and a target.
Cloud base: Height of the lowest cloud layer above the surface.
Coast Pilot (US): NOAA equivalent of Sailing Directions covering US waters.
Coastal navigation: Position-fixing within sight of land using visual and radar methods.
Coastal waters: Waters within twelve nautical miles of the coast.
Cocked hat: Triangle formed by three intersecting position lines.
COLREGs: Convention on the International Regulations for Preventing Collisions at Sea, 1972; the IMO rules of the road governing lights, shapes, sound signals, and collision avoidance.
Compass adjustment: Process of minimizing deviation by placing correctors.
Compass bearing: Bearing taken from the magnetic or gyro compass.
Compass Card: Graduated card of the magnetic compass.
Compass course: Course referenced to compass north.
Compass error: Combined deviation and variation.
Compass north: Direction indicated by the compass needle.
Compass Rose: Wind rose device printed on charts since the portolan era of the late thirteenth century.
Composite great circle: Track combining great-circle and limiting-latitude rhumb-line segments.
Confused sea: Sea with no dominant direction, often from interacting swell systems.
Conic projection: Chart projection using a cone tangent or secant to the Earth.
Constants of the place: Tidal harmonic constants for a specific port.
Continuous Synopsis Record (CSR): Required under SOLAS XI-1/5.
Convergence (wind): Net inflow of air into a region, associated with rising air and low pressure.
Coordinated Universal Time (UTC): International time standard used in navigation.
Coriolis force: Apparent force due to Earth's rotation, deflecting moving objects.
Correction (compass): Procedure to convert compass course to true course.
Cospas-Sarsat: International satellite system for 406 MHz distress beacon detection.
Course: Direction in which the vessel is steered.
Course made good (CMG): Resultant track of the vessel over ground.
Course over ground (COG): Vessel's actual course relative to the Earth as measured by GNSS.
Course recorder: Device logging compass heading over time.
Course to steer (CTS): Course required to make good a desired track allowing for set and drift.
Course-up display: Radar/ECDIS orientation with intended course at the top.
Crepuscular: Pertaining to twilight conditions.
Crepuscular twilight: See civil, nautical, astronomical twilight.
Cross bearing fix: Position determined by intersection of two or more visual bearings.
Cross-track distance (XTD): Perpendicular distance from planned route to vessel position.
Cross-track error (XTE): Synonym for XTD in GNSS/ECDIS context.
Culmination: Passage of a body across the observer's meridian.
Current: Horizontal movement of water; ocean current is permanent, tidal stream is periodic.
Cut: Angle of intersection of two position lines.
Cyclone: Low-pressure rotating storm system; in tropics called a typhoon or hurricane depending on basin.

D

Dangerous quadrant: Forward half of the dangerous semicircle.
Dangerous semicircle: Right-hand semicircle of a TRS in the Northern Hemisphere and left-hand in the Southern, where winds drive a vessel into the storm track.
Datum (chart): Reference plane for soundings; see Chart datum.
Datum (horizontal): Reference ellipsoid for positions, e.g., WGS-84.
Day's run: Distance traveled in 24 hours, noon to noon.
Daylight saving time: Civil time adjustment; not used in nautical chronometry.
Daymark: Distinctive shape hoisted in daylight to indicate a vessel's status.
Daytona (no entry): Not a navigational term.
DCPA: Distance at the closest point of approach (synonym for CPA).
Dead reckoning (DR): Position by course and speed without external fix.
Decca: Obsolete hyperbolic radio-navigation system, decommissioned 2000.
Declination: Angular distance of a body north or south of the celestial equator.
Deep-draught route: Charted route surveyed for vessels of deep draught.
Departure (navigation): Distance in nautical miles measured east or west along a parallel.
Depression (meteorology): Low-pressure system.
Deviation: Departure from the contractual or geographic voyage; historically a fundamental breach.
DGPS (Differential GPS): GPS augmentation using ground reference stations for corrections.
Digital Selective Calling (DSC): GMDSS technique on VHF Ch 70, MF 2187.5 kHz, HF, for distress alerting.
Dip (horizon): Angle between sensible horizon and visible horizon, depending on height of eye.
Dip (magnetic): Vertical angle the Earth's magnetic field makes with the horizontal.
Distress alert: GMDSS message indicating grave and imminent danger.
Distress signals: Signals listed in COLREGs Annex IV indicating distress and need of assistance.
Diurnal Tide: Tide with one high and one low water per tidal day.
Divergence (wind): Net outflow of air, associated with subsidence and high pressure.
Dividers: Drafting instrument with two metal points for measuring distance on a chart.
Dogleg: Sudden change in course along a track.
Doldrums: Equatorial region of light and variable winds within the ITCZ.
Doppler log: Speed log using Doppler shift of reflected acoustic signals.
Douglas protractor: Square transparent plotting protractor.
Draft (chart): Reference to draught used in under-keel-clearance calculations.
Drag (anchor): Failure of an anchor to hold, allowing the vessel to move.
Drift: Speed of a current; leeway of a vessel.
Drift angle: Angle between the ship's head and her track during a turn.
Drying height: Height above chart datum of a feature that uncovers at low water.
DSC controller: Built-in or standalone DSC modem.
Dutchman's log: Crude speed measurement by timing a floating object along a known ship length.

E

Earth's magnetic field: Geomagnetic field with poles displaced from geographic poles.
Easterly variation: Magnetic variation where magnetic north lies east of true north.
EasyTide: Free UKHO online seven-day tidal-prediction service (limited port set).
Ebb: Outgoing tide.
ECDIS: Electronic Chart Display and Information System per IMO MSC.232(82).
Echo sounder: Acoustic depth sensor.
Eclipse (light): Period during which a flashing or occulting light is not visible.
Ecliptic: Plane of Earth's orbit around the Sun; great circle on the celestial sphere.
ECS (Electronic Chart System): Non-IMO-compliant electronic chart display.
EGC (Enhanced Group Call): Inmarsat-C broadcast service for MSI and SafetyNET.
Electronic Bearing Line (EBL): Radar feature for measuring bearing to a target.
Electronic Navigational Chart (ENC): S-57 vector chart format.
Elevation (geographical): Height of an object above sea level.
eLoran: Enhanced terrestrial radio-navigation system using improved Loran-C signals.
Embarkation point: Point from which a survey or pilot departs the vessel.
Engine telegraph: Bridge device for ordering engine speed.
Ephemeris: Tabulated celestial positions.
EPIRB (Emergency Position-Indicating Radio Beacon): 406 MHz Cospas-Sarsat beacon transmitting distress alerts.
Equation of time: Difference between mean and apparent solar time.
Equator: Great circle equidistant from the geographic poles.
Equinox: Vernal/autumnal crossing of the celestial equator.
Equipment carriage requirements: Navigational equipment required by SOLAS Chapter V Regulation 19.
Error (compass): Algebraic difference between true and compass heading.
Error budget: Total allowable position error from all sources.
Estimated position (EP): DR position corrected for set and drift.
Estimated Time of Arrival (ETA): Predicted time of arrival.
Estimated Time of Departure (ETD): Predicted departure time from a port.
Execution stage: Third stage of passage planning per IMO Resolution A.893(21).
Extreme low water: Lowest water level recorded at a port.
Eye (TRS): Calm low-pressure center of a tropical revolving storm.
Eyewall: Ring of intense convection surrounding the eye of a TRS.

F

Fairway: Navigable channel.
False echo: Spurious radar return from side lobes, multiple reflections or interference.
Fathom: Unit of depth of 6 feet, 1.8288 m.
Featherweight (sextant): Lightweight sextant model.
Ferry glide: Maneuver using current to move sideways relative to the ground.
Fetch: Distance over water across which the wind blows generating waves.
First point of Aries: Origin of right ascension and sidereal hour angle on the celestial equator.
Fix: Determined geographical position.
Fixed light: Light showing a steady beam.
Flag signal: Visual communication using International Code of Signals flags.
Flash (light): Light phase in which duration of light is less than duration of darkness.
Fleet Broadband: Inmarsat IP and voice service over FB150/250/500 terminals.
Floe: Individual piece of sea ice.
Flood: Incoming tide.
Fluxgate compass: Magnetic compass using a fluxgate sensor.
Fog: Suspension of water droplets reducing visibility below 1 km.
Fog signal: Audible signal under COLREGs Rule 35 in restricted visibility.
Following sea: Sea running in the same direction as the ship.
Foreign-going voyage: Voyage between ports of different countries.
Form NP133A: Chart correction log (Admiralty).
Frequency band: Allocated portion of the radio spectrum.
Front (meteorology): Boundary between two air masses of different properties; cold, warm, occluded or stationary.

G

Galileo: EU GNSS constellation.
General chart: Small-scale chart for ocean passages.
Geographical position (GP): Point on Earth directly beneath a celestial body.
Geographical range: Distance to the geographic horizon from a given height.
Geoid: Equipotential surface of Earth's gravity field approximating mean sea level.
Geomagnetic pole: Axis of the best-fit dipole to Earth's magnetic field.
GHA (Greenwich Hour Angle): Angle measured westward from the Greenwich meridian to the hour circle of a body.
GLONASS: Russian GNSS constellation.
GMDSS (Global Maritime Distress and Safety System): SOLAS communications framework.
GMT (Greenwich Mean Time): Former name for the time scale now expressed as UT or UTC.
Gnomonic chart: Great-circle-as-straight-line chart projection.
GNSS (Global Navigation Satellite System): Generic term for GPS, GLONASS, Galileo, BeiDou and SBAS.
GPS: Global Positioning System, US satellite navigation system.
Gradient wind: Wind resulting from balance of pressure-gradient, Coriolis and centripetal forces.
Great circle: Circle on a sphere passing through the center.
Great circle sailing: Method of computing the shortest track between two points.
Greenwich meridian: Prime meridian at 0 degrees longitude.
Grid course: Course measured from grid north on polar charts.