Marine Navigation – Core Vocabulary

These vocabulary flashcards cover fundamental equipment, publications, calculations, navigational terms, and radar concepts essential for marine navigation study and exam preparation.

Navigation

The science of monitoring and controlling a ship’s movement from one place to another along the safest, most economical route.

Gyro Compass

An electrically-powered compass that aligns with true north using gyroscopic inertia and the Earth’s rotation, unaffected by magnetism.

Magnetic Compass

A compass that aligns with magnetic north by responding to the Earth’s magnetic field; requires no electricity.

Radar

Electronic equipment that transmits radio waves and displays echoes to detect and track nearby vessels, land, and hazards.

Echo Sounder

Instrument that sends sound pulses to the seabed and measures the time until return to display depth under the keel.

Ship’s Log (instrument)

A device fitted in the hull to measure a vessel’s speed and distance travelled; data shown on the bridge display.

Global Positioning System (GPS)

Satellite-based navigation system that provides real-time vessel position, course, and speed anywhere on Earth.

Automatic Identification System (AIS)

Short-range transponder system that broadcasts a ship’s identity, position, course, and speed to other ships and shore stations.

Electronic Chart Display and Information System (ECDIS)

Computerized navigation system that displays official digital charts with real-time GPS plots and optional radar overlay.

Radio Direction Finder (RDF)

Medium-frequency receiver with a direction finder used to obtain bearings of transmitting radio stations or ships.

Chart

A specialized map for marine navigation, showing depths, hazards, aids to navigation, and coastal features.

Mercator Projection

Chart projection where meridians are parallel; rhumb lines plot as straight lines, widely used for navigation.

Gnomonic Projection

Chart projection on which great circles plot as straight lines; useful for planning long-distance great-circle routes, especially near the poles.

Sailing Directions (Pilots)

Publications giving detailed coastal information on weather, tides, dangers, ports, and local regulations.

Tide Tables

Publications predicting times and heights of high and low water at specific ports, including moon phases and daylight data.

Nautical Almanac

Annual publication providing astronomical data used with a sextant for celestial navigation.

Nories Tables

Reference tables used for course and distance calculations and for determining celestial azimuths.

List of Lights and Radio Signals

Publication listing lighthouse characteristics and radio station details such as frequencies and operating schedules.

Notices to Mariners

Regular updates that correct charts and publications and provide navigational warnings.

Chronometer

High-precision marine clock used to determine longitude by comparing local time with Greenwich Mean Time.

Azimuth Ring

360° ring fitted on a compass for taking bearings through a prism and V-sight.

Azimuth Mirror

Instrument with a black mirror that reflects the sun to reduce glare when taking high-altitude sights.

Parallel Rulers

Two linked straightedges that slide together for transferring bearings and courses on charts.

Dividers

Two-legged tool used to measure distances on charts via the latitude scale.

Great Circle

The largest possible circle on a sphere; represents the shortest distance between two surface points.

Small Circle

Any circle on a sphere that does not pass through the centre, e.g., parallels of latitude (except the equator).

Equator

The great circle midway between the poles at 0° latitude.

Meridian

A semi-great circle running from pole to pole; reference for measuring longitude.

Prime Meridian

The meridian through Greenwich (0° longitude) used as the reference for east–west measurements.

Latitude

Angular distance north or south of the equator, measured along a meridian.

Longitude

Angular distance east or west of the Prime Meridian, measured along parallels.

Difference of Latitude (d.lat)

Angular difference between two latitudes; subtract if same hemisphere, add if opposite.

Difference of Longitude (d.long)

Angular difference between two longitudes; subtract if same side of Greenwich, add if opposite.

Nautical Mile

Standard sea-distance equal to 1,852 m or one minute of latitude.

Sea Mile

One minute of arc along a meridian; slightly varies with latitude.

Geographical Mile

One minute of longitude at the equator (≈ 1,854.6 m).

Knot

Unit of speed at sea equal to one nautical mile per hour.

Cable

Distance unit equal to one-tenth of a nautical mile (≈ 185 m).

Bearing

Direction of an object expressed in degrees clockwise from north (true, magnetic, or compass).

True Bearing

Bearing measured relative to true north.

True Course

Intended direction of a vessel through still water, referenced to true north.

Rhumb Line

Line crossing all meridians at the same angle; plots straight on a Mercator chart.

Departure (navigation)

East–west distance between two points along a rhumb line, calculated from d.long × cos (latitude).

Passage Planning (PECAN)

Structured process of Preparation, Execution, Control, and Anticipation with required Notation for every voyage.

Preparation (PECAN)

Phase of gathering charts, publications, fuel, provisions, and identifying dangers before departure.

Execution (PECAN)

Phase in which the approved voyage plan is put into action and the crew briefed.

Control (PECAN)

Continuous monitoring of progress and safety while underway.

Anticipation (PECAN)

Foreseeing and preparing for potential problems during the voyage.

Notation (PECAN)

Mandatory recording of voyage details in the ship’s logbook.

Variation (Declination)

Angular difference between true and magnetic north at a location.

Deviation

Error of a magnetic compass caused by the vessel’s own magnetism.

Swinging the Compass

Procedure of turning a vessel through headings to measure and correct compass deviation.

Deviation Card

Table listing residual compass errors on each heading after adjustment.

Dead Reckoning (DR)

Estimating future position from current position, course, and speed without external references.

Estimated Position (EP)

DR position corrected for predicted wind, current, and other effects.

Fix

Exact vessel position obtained from at least two simultaneous position lines.

Set

Direction toward which a current flows, expressed in degrees from north.

Drift

Speed of a current, usually in knots.

Leeway

Sideways movement of a vessel caused by wind.

Running Fix

Position determined by advancing an earlier line of position to intersect a later one.

Transit

Two or more fixed objects in line used to check or obtain a position.

Range (distance)

Distance between two points, normally expressed in nautical miles.

Pilotage

Navigation by visual reference to coastal landmarks and aids.

Sounding

Measuring water depth with a lead line or echo sounder.

Tide

Regular rise and fall of sea level due to gravitational forces of the moon and sun.

Current

Horizontal movement of water.

Parallel Indexing

Radar method using a line parallel to own course to maintain a safe distance from hazards.

COLREGS

International Regulations for Preventing Collisions at Sea.

Chart Datum

Reference vertical level from which charted depths are measured.

Depth Contour

Line on a chart connecting points of equal depth below chart datum.

Light Characteristics

Distinct flashing pattern of a navigational light used for identification.

Buoyage

System of buoys and beacons marking channels and hazards.

IALA

International Association of Marine Aids to Navigation and Lighthouse Authorities; sets buoyage standards.

Safe Water Mark

Navigation mark indicating safe water all around it (red and white vertical stripes).

Isolated Danger Mark

Mark placed over a single hazard with navigable water all around (black with red bands, two black balls).

Special Mark

Yellow navigation mark indicating a special area or feature described in charts/publications.

Cardinal Mark

Mark that indicates safest water lies to the north, east, south, or west of it.

Range Lights

Two fixed lights positioned so their alignment marks a safe channel.

Sector Light

Light showing different colours over specific bearings to indicate safe sectors.

Bridge (ship)

Area from which a ship is commanded and navigated.

Conning

Giving helm and engine orders to control course and speed.

Steering Gear

Machinery and hydraulic systems that move the rudder.

Helm

Wheel or lever used by the helmsman to steer the vessel.

Underway

Vessel not at anchor, aground, or otherwise secured to shore.

Making Way

Vessel moving through the water with steerageway.

Drifting (vessel)

Underway but not under control; moved only by wind or current.

Swinging Circle

Area a ship can occupy while swinging at anchor (ship length + cable length).

Drag (anchor)

Condition where the anchor fails to hold and the vessel moves.

Let Go Anchor

Command to release the anchor so it drops to the seabed.

Weigh Anchor

Operation of raising the anchor from the seabed.

Scope

Ratio of anchor cable length to water depth.

Short Stay

When anchor cable is nearly vertical just before heaving up.

Anchor Aweigh

Anchor is clear of the seabed but not yet secured on deck.

Fouled Anchor

Anchor caught on an obstruction or its own cable.

Bitter End

Inboard end of the anchor cable secured in the chain locker.

Admiralty Chart

Official chart produced by the UK Hydrographic Office.

Small Craft Chart

Chart with detailed inshore information intended for smaller vessels.

Chart Correction

Process of updating charts with latest information from Notices to Mariners.

Light List

Publication describing characteristics and positions of navigational lights.

Pilot Book

Publication with detailed coastal navigation, anchorage, and regulation information.