Glossary — navigation

Charts, position-fixing, watchkeeping, COLREG. Bridge-side terminology — celestial, electronic, coastal.

59 terms in this category.

• Automatic Identification System (AIS)
  VHF transponder system that automatically broadcasts vessel identity, position, course, and speed to nearby ships and shore stations.
• Azimuth
  The angular bearing of a celestial body measured clockwise from true north, used to check compass error.
• Bearing
  The horizontal direction of one point from another, expressed in degrees from 000° to 359°.
• Bergy Bit
  Medium fragment of glacier ice, 1–5 m above water and 5–15 m across.
• Celestial Navigation
  The art and science of determining a vessel's position by measuring the altitudes of celestial bodies with a sextant.
• Chart Datum
  The reference tidal level from which chart depths + drying heights are measured — typically Lowest Astronomical Tide (LAT) in modern charts.
• Chronometer
  A precision timekeeping instrument used in celestial navigation to provide accurate GMT for the calculation of longitude.
• Clearing Line
  A bearing or line on the chart that, when maintained, keeps a vessel clear of a specific hazard such as a shoal or reef.
• COLREG Rule 13 — Overtaking
  Any vessel overtaking another shall keep out of the way of the vessel being overtaken — applies regardless of relative status under other rules.
• COLREG Rule 14 — Head-On
  When two power-driven vessels meet on reciprocal courses, each shall alter course to starboard.
• COLREG Rule 15 — Crossing
  When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way.
• COLREG Rule 18 — Responsibilities
  Hierarchy of stand-on / give-way: power-driven vessels give way to sailing, sailing to fishing, fishing to NUC and RAM.
• COLREG Rule 19 — Restricted Visibility
  Conduct of vessels in or near restricted visibility — early action, safe speed, and avoidance of port-side alterations to a vessel forward of the beam.
• COLREG Rule 5 — Lookout
  Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate.
• COLREG Rule 6 — Safe Speed
  Every vessel shall proceed at a safe speed so she can take proper and effective action to avoid collision.
• COLREG Rule 7 — Risk of Collision
  Every vessel shall use all available means to determine if risk of collision exists; if in doubt, such risk shall be deemed to exist.
• Compass
  Instrument indicating direction relative to magnetic north or true north, fundamental to ship navigation.
• Compass Bearing
  A bearing measured by a ship's compass, requiring correction for both variation and deviation to obtain a true bearing.
• Compass deviation
  The error of the magnetic compass caused by the ship's own magnetic field — corrected via periodic swinging + adjustment.
• CPA (Closest Point of Approach) (CPA)
  The minimum distance between own ship + a target — a primary radar/ARPA collision-risk indicator.
• Danger Angle
  A horizontal or vertical sextant angle that, when not exceeded, ensures a vessel remains clear of a coastal hazard.
• Dead Reckoning (DR)
  Calculating current position using a known past position, then advancing it using speed, course, and elapsed time.
• Deviation
  The error in a magnetic compass caused by the ship's own magnetic field, varying with the vessel's heading.
• Deviation Card
  A tabulated record of a magnetic compass's deviation on successive headings, prepared after compass adjustment or swing.
• ECDIS (Electronic Chart Display + Information System) (ECDIS)
  The mandatory digital nautical chart display system on SOLAS ships — replaces or supplements paper charts.
• Electronic Chart Display and Information System (ECDIS)
  A navigation information system that displays ENCs and vessel position, accepted as a replacement for paper charts under SOLAS.
• Estimated Position (EP)
  A position obtained by advancing a known fix using course and speed while applying estimated leeway and current allowances.
• Fix
  A confirmed vessel position established by two or more simultaneous position lines from observed bearings, ranges, or electronic navaids.
• Global Positioning System (GPS)
  US satellite-based radionavigation system providing position, velocity, and time data worldwide to marine and other users.
• Great Circle
  The shortest route between two points on a sphere, formed by the intersection of the sphere and a plane through its center.
• Growler
  Small piece of glacier ice (<1 m above water, ~5 m across) — particularly dangerous because it is hard to detect on radar.
• Gyrocompass
  A compass using a gyroscope to find geographic north, unaffected by magnetic fields; mandatory on most large vessels.
• Iceberg
  Mass of glacier ice >5 m above the water, classified small/medium/large/very large by IIP.
• Intercept
  In celestial navigation, the difference in minutes of arc between the observed altitude and the calculated altitude, used to plot a position line.
• Lead
  Linear crack or fracture in sea ice large enough for navigation.
• Leading Line
  Two charted objects in line that define a safe course into or within a port, keeping the vessel clear of hazards.
• Loxodrome
  A line on the earth's surface crossing all meridians at the same angle, synonymous with a rhumb line course.
• Magnetic Bearing
  A bearing measured clockwise from magnetic north, requiring correction for variation to obtain a true bearing.
• Magnetic Compass
  A compass that aligns with Earth's magnetic field to indicate magnetic north; required as backup on all SOLAS vessels.
• Mercator Sailing
  A method of solving rhumb line problems using the properties of a Mercator chart to find course and distance between positions.
• Northern Sea Route (NSR)
  Russian Arctic shipping route along Siberia from Kara Gate to Bering Strait; transits regulated by Glavsevmorput (NSR Administration).
• Not Under Command (NUC)
  Vessel which through exceptional circumstance cannot manoeuvre as required by the rules; shows two black balls / two red lights vertically.
• NUC (Not Under Command) (NUC)
  COLREG Rule 3(f) status where a vessel is unable to manoeuvre due to exceptional circumstances (steering failure, engine failure) and cannot keep out of the way of another vessel.
• Parallel Index (PI)
  A radar technique using lines parallel to the course line to monitor closest approach to a fixed radar target.
• Polynya
  Persistent area of open water surrounded by sea ice, shaped by wind/current or upwelling.
• Restricted in Ability to Manoeuvre (RAM)
  Vessel which from the nature of her work is restricted in her ability to manoeuvre; shows ball-diamond-ball / R-W-R lights.
• Rhumb Line
  A course that crosses all meridians at the same angle, appearing as a straight line on a Mercator chart.
• Running Fix
  A position obtained from two or more bearings of the same object taken at different times and advanced along the course line.
• Sextant
  A precision optical instrument used to measure the angle between a celestial body and the horizon for position fixing.
• Sight Reduction
  The mathematical process of converting a celestial observation into a position line using tables or a calculator.
• TCPA (Time to Closest Point of Approach) (TCPA)
  The time remaining until own ship + a target reach their Closest Point of Approach — the primary time-window indicator for collision-avoidance action.
• Three-Point Bearing
  A position fix obtained by simultaneously measuring horizontal angles between three charted objects using a sextant or compass.
• Transit
  Two or more objects in line with each other, used as a position line or to verify vessel alignment in a channel.
• True Bearing
  A bearing measured clockwise from true north, used for chart plotting and unaffected by magnetic variation or compass deviation.
• Under Keel Clearance (UKC) (UKC)
  The vertical distance between the ship's deepest point + the seabed — a company-set safety-margin parameter for shallow-water navigation.
• Variation
  The angular difference between true north and magnetic north at a given location, shown on navigational charts.
• Voyage Data Recorder (VDR)
  A tamper-proof device recording bridge audio, radar images, and navigational data onto a protected capsule for post-incident investigation.
• Weather Routing
  A service providing vessel operators with recommended routes based on forecast weather and sea conditions to optimise safety and economy.
• Williamson turn
  Man-overboard ship-turning manoeuvre — helm hard-over one way, then the opposite, returning the ship to its own wake and the casualty's approximate position.