| term | meaning (click term for explanation in text) |
| astro(nomical) navigation | navigating by taking sights on the sun, moon, planets and stars |
| bearing | the direction from one object (or observer) to another |
| cable | unit of distance: one tenth of a nautical mile (185m) |
| chart | map for navigating at sea |
| chart datum | precise definition of latitude and longitude for a particular chart |
| clearing line | a line on a chart which has safe water to one side of it |
| cocked hat | the triangular “box” formed by three position lines, e.g. using a three bearing fix |
| course | a) the direction the boat is being steered in b) a line on a chart showing where you want the boat to go |
| course steered | the direction the boat is steered in |
| dead reckoning DR | position worked out by course and distance logged |
| Decca | an electronic device for navigation - obsolescent |
| degrees | measurement of angle: 360 degrees is a full circle |
| depth contour | a line printed on a chart showing places of equal depth |
| deviation | see magnetic deviation |
| differential GPS - DGPS | GPS corrected by position error transmitted by a local reference station |
| dipping | an object or light just disappearing below the observer’s horizon |
| distance off | the distance from the boat to the coast, light or landmark |
| estimated position EP | position worked out by course and distance logged, plus estimate of leeway and tidal stream |
| Global Position System GPS | an electronic position fixing system using satellites |
| gross error | error caused by mistake (as opposed to inaccuracy) |
| ground track | a) the direction travelled by the boat relative to the ground (as opposed to through the water) b) a line drawn on a chart showing the motion relative to the ground |
| heading | the direction the boat is pointing in |
| knot | measurement of speed: one nautical mile per hour |
| latitude | angular distance north or south of the equator |
| leeway | the difference between the heading (or course steered) and the water track |
| log | a) instrument for measuring distance travelled through the water b) written record of navigational information |
| longitude | angular distance east or west of the Greenwich Meridian |
| loom | the light in the sky from a lighthouse which is just below the observer’s horizon |
| Loran | an electronic navigation system similar to Decca |
| magnetic | directions referred to magnetic north as opposed to true north |
| magnetic compass | navigation instrument that shows the direction steered by the ship or the bearing of an object |
| magnetic deviation | error in ship’s magnetic compass caused by the hull and metal objects |
| magnetic north | the direction a magnetic compass indicates as north: towards the magnetic north pole |
| magnetic variation | difference between true north and magnetic north |
| meridian | a north-south line on the chart (where midday occurs at the same time for every point on the line) |
| minutes | angular measurement: 60 minutes equals one degree |
| nautical mile | measurement of distance: one minute of latitude or 1852m |
| nautical mile per hour | measurement of speed - same as knot |
| plot | mark on the chart |
| points of the compass | subdivisions between North, East, South and West |
| position fix | determination of the boat’s position |
| position line | a line drawn on a chart, such that the boat must lie somewhere on that line |
| radio direction finding RDF | determining the bearing of a radio transmission (to draw a position line) |
| rising | an object or light just appearing above the observer’s horizon |
| seconds | angular measurement: 60 seconds equals one minute (obsolete) |
| sectored light | a navigation light which shows different colours or characteristics in different directions |
| sextant | a device for making an accurate measurement of angle between two observed objects |
| three bearing fix | technique for fixing the boat’s position by taking bearings on three objects |
| track | see water track, ground track |
| transferred position line | position line moved through a distance equal to the estimated movement of the boat |
| transit | two objects observed to be in line |
| true directions | referred to true north as opposed to magnetic north |
| true north | towards the North pole (defined by the earth’s axis of spin) |
| variation | see magnetic variation |
| vertical sextant angle | measurement of the vertical angle between two objects, using a sextant |
| water track (aka wake track) | a) the direction travelled by the boat through the water (as opposed to relative to the ground) b) a line drawn on a chart showing the path through the water |