| 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 |
| current | general movement of water in an ocean (not tidal stream) |
| 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 |
| term | meaning (click term for explanation in text) |
| chart datum | height datum to which charted depths are referred |
| charted depth | depth marked on chart |
| diurnal | once a day - a tidal pattern repeating every 24 hours approx |
| drying height | height above chart datum of a point which is sometimes submerged and sometimes dries |
| duration | elapsed time from low water to the following high water |
| ebb | tide going out |
| flood | tide coming in |
| height of tide | height of water above LAT chart datum |
| high water HW | tide fully in, maximum depth of water |
| low water LW | tide fully out, minimumdepth of water |
| lowest astronomical tide LAT | minimum sea level ever expected at low water (in normal atmospheric conditions) |
| mean high water neaps MHWN | mean (average) level of high water, for neap tides |
| mean high water springs MHWS | mean (average) level of high water, for spring tides |
| mean low water neaps MLWN | mean (average) level of low water, for neap tides |
| mean low water springs MLWS | mean (average) level of low water, for spring tides |
| neap range | difference between HW and LW heights during neap tides |
| neap tide | smaller tide, which occurs just after the first and third quarters of the moon |
| range | height difference between successive HW and LW |
| secondary port | a port for which there is no tide table in the almanac |
| semi-diurnal | twice a day - a tidal pattern repeating every 12 hours approx (i.e. twice in 24 hours) |
| slack | period of zero tidal stream |
| spring range | difference between HW and LW heights during spring tides |
| spring tide | larger tide which occurs just after new moon and full moon |
| standard port | port for which full tide tables are published in the almanac |
| stand | period when the water level neither rises nor falls |
| tidal curve | published curve which allows you to predict heights of tide between HW and LW |
| tidal diamond | chart symbol indicating a position for which tidal stream data is provided |
| tidal stream | motion of water caused by flooding or ebbing tide |
| tidal stream atlas | publication which predicts tidal streams for an area, printed on small charts |
| tide table | publication of the times and heights of high and low water |
| universal time UT | standard global time datum (same as GMT for our purposes). Also known as Universal Time Coordinated (UTC) |
| term | meaning (click term for explanation in text) |
| cardinal mark | family of marks in the IALA buoyage system |
| chart datum | survey datum to which charted positions (latitude, longitude) are referred (same term is used for height datum to which depth are referred) |
| chart plotter | device which displays charts on a screen |
| chart 5011 | Admiralty publication: Symbols and Abbreviations Used on Admiralty Charts |
| daymark | an unlit structure built (and charted) to assist daytime navigation |
| direction light | a light which show in a particular direction |
| east cardinal | an IALA buoyage system mark in the cardinal mark family |
| fixed | a light showing on continuously |
| general direction of buoyage | the direction in which channels are defined by keeping port-hand marks to port and starboard-hand marks to starboard |
| height datum | datum to which charted heights above sea level are referred (usually MHWS) |
| IALA buoyage system | an internationally agreed convention for navigation marks, defining what they look like and how they are used |
| interrupted | a flashing light which stops flashing for a time them resumes |
| isolated danger mark | a mark in the IALA buoyage system showing an isolated danger |
| isophase | a light which shows alternately on and off for equal times |
| large scale | chart which shows a small geographical area in great detail |
| lateral mark | family of marks in the IALA buoyage system |
| light characteristics | the pattern and colour of a light, as described on a chart |
| luminous range | the brightness of a light, expressed as its nominal visible range in good visibility |
| mercator | a projection (q.v.) which is commonly used for marine charts |
| navigation | (see pilotage) set of techniques of getting from A to B, off-shore and not close to dangers |
| navigational aids | marks, lights and buoys placed to assist in navigation and pilotage |
| north cardinal | an IALA buoyage system mark in the cardinal mark family |
| occulting | a light which shows mainly on and goes off for short periods (the on periods are longer than the off periods) |
| period | the length of time it takes for a light pattern to repeat itself |
| pilotage | (see navigation) set of techniques for getting into or out of harbour and navigating close to dangers |
| port-hand lateral mark | an IALA buoyage system mark in the lateral mark family |
| preferred channel mark | an IALA buoyage system mark in the lateral mark family |
| projection | the technique used for representing the curved surface of the earth on a flat paper chart |
| region A | geographical area where IALA lateral marks are defined in a particular way (different from Region B) |
| region B | geographical area where IALA lateral marks are defined in a particular way (different from Region A) |
| safe water mark | a mark in the IALA buoyage system showing safe water or the entrance to a channel |
| scale | ratio between distance represented and distance on the chart |
| sectored light | a light which shows different colours in different directions |
| small scale | a chart which shows a large area with little detail of ports etc |
| south cardinal | an IALA buoyage system mark in the cardinal mark family |
| special mark | a mark defined in the IALA buoyage system which is placed for another purpose and is not a navigational aid |
| starboard-hand lateral mark | an IALA buoyage system mark in the lateral mark family |
| transit marks | marks which are arranged to line up guide vessels, e.g. into and out of a harbour |
| transverse mercator | a projection (q.v.) which is commonly used for marine charts |
| west cardinal mark | an IALA buoyage system mark in the cardinal mark family |
| WGS 84 | World Geodetic System 1984 - a chart surveying datum which is becoming standard because of ite compatibility with GPS |
| term | meaning (click term for explanation in text) |
| all round light | light on a vessel that shines in a full circle (white, red or green) |
| collision, risk of | see risk of collision |
| COLREGS | another name for International Regulations for Prevention of Collision at Sea |
| constrained by draught | vessel restricted in the course she can follow because of the available depth of water |
| engaged in fishing | vessel which is actively engaged in fishing |
| fishing | see engaged in fishing |
| flashing light | light on a vessel that flashes with a frequency of 120 or more per minute |
| give-way vessel | vessel that is required to give way in a situation where there is a risk of collision |
| IMO | International Maritime Organisation |
| IRPCS | International Regulations for Prevention of Collision at Sea |
| masthead light | light on a vessel that shines white in a forward sector |
| mayday | codeword for distress; message of distress |
| not under command | vessel unable to manoeuvre because of some exceptional circumstance |
| overtaking | coming up from behind another vessel to go past it |
| port tack | sailing with the wind on the port side, or with mainsail set on the starboard side |
| restricted in ability to manoeuvre | vessel unable to manoeuvre because of her work |
| risk of collision | two vessels approaching one another on a constant bearing |
| sidelight | light on a vessel that shines red in port sector or green in starboard sector |
| stand-on vessel | vessel that is required to hold her course and speed in a situation where there is a risk of collision |
| starboard tack | sailing with the wind on the starboard side, or with mainsail set on the port side |
| steaming light | see masthead light |
| sternlight | light on a vessel that shines white in a sector astern |
| towing light | light on a vessel that shines yellow in a sector astern |
| traffic lane | defined lane in a traffic separation scheme for vessels proceeding in one direction |
| traffic separation scheme (TSS) | area marked on the chart where special IRPCS rules apply because of volume of shipping traffic |
| types of light | defined types of light on vessels (masthead light, sidelights, sternlight etc) |
| term | meaning (click term for explanation in text) |
| acceleration zone | area subject to increased wind because of the way the air flows round high land |
| advection | see fog, advection |
| air mass | term used to describe a large region of atmoshperic air with similar properties, e.g. wet, dry, warm, cold |
| anticyclone | an area of high pressure |
| Atlantic depression, Atlantic low | a frontal low pressure system that forms in the north Atlantic - the cause of much wind and rain in the UK and Ireland |
| backing | wind changing direction in an anticlockwise sense e.g. west to south |
| bar | unit of pressure approximately equal to the average atmospheric pressure: 1000 milibars |
| Beaufort scale | standard scale for wind strength |
| Buys Ballot's Law | law of meteorology which describes the relationship between wind and atmoshperic pressure |
| cold front | a front in which a cold air mass is "pushing into" a warm one, giving particular weather characteristics |
| convection | vertical movement of air due to temperature: warm air rising |
| coriolis | effect which causes objects/fluids to move in a curved line on the earth, due to its rotation |
| cumulonimbus Cb | type of cloud: towering cumulus with rain or hail showers |
| cumulus Cu | type of cloud: isolated, low level fluffy clouds, caused by convection |
| cyclonic | wind changing direction due to close passage of the centre of a depression |
| depression | see Atlantic depression |
| fetch | distance the wind blows over sea to build up waves |
| fog | term for visibility of less than 1000m |
| fog, advection | fog which forms when "wet" air blows over a cold surface |
| fog, radiation | fog which forms when land cools down at night under clear sky |
| front | weather pattern caused by two air masses meeting - usually part of a low pressure system |
| good (visibility) | visibility greater than 5 miles |
| gradient wind | general (as opposed to local) wind caused by the atmospheric pressure distribution |
| hectopascal | unit of pressure equal to milibar |
| high (pressure) | see anticyclone |
| high, very high (sea state) | sea states with greater than 6m wave height |
| imminent | within 6 hours of the issue time of the forecast/warning |
| inshore waters forecast | forecast service from UK Met Office, for up to 12 miles offshore |
| isobar | line on a weather map joining places of equal atmospheric pressure |
| katabatic wind | wind caused by cold air flowing down from high ground |
| land breeze | local off-shore wind near coast at night, caused by air cooling and sinking over land |
| later | more than 12 hours from the issue time of the forecast | low | see Atlantic depression |
| Marinecall | UK weather forecasting service |
| milibar | unit of atmospheric pressure, approx one thousandth of average atmospheric pressure |
| moderate (sea state) | sea state with wave height between 1.25m and 2.5m |
| moderate (visibility) | visibility between 2 and 5 miles |
| occlusion, occluded front | type of front where the warm air mass is no longer in contact with the surface |
| phenominal (sea state) | something you probably never want to experience: wave height in excess of 14m |
| poor (visibility) | visibility between 2 miles and 1000m |
| pressure gradient | area of changing atmospheric pressure (analogous to sloping ground) |
| ridge | elongated area of high pressure |
| rough, very rough | sea states: wave height 2.5m to 4m; and 4m to 6m |
| sea breeze | local wind near the coast, caused by warm air rising over the land |
| sea state | description of how rough the sea is: wave height in open water |
| secondary depression | a depression that forms along a front from another depression |
| slight | sea state with wave height from 0.5m to 1.25m |
| smooth | sea stath with wave height less than 0.5m |
| soon | between 6 and 12 hours from time of issue of forecast/warning |
| synoptic chart | weather chart showing isobars and fronts |
| unstable atmosphere | condition when upper air is particularly cold, favouring the development of cumulonimbus clouds and heavy showers |
| veering | wind changing direction in a clockwise sense e.g. west to north |
| vortex | circular movement of fluid round extreme low pressure (e.g. plug hole in bath!) |
| warm front | a front in which a warm air mass is "pushing into" a cold one, giving particular weather characteristics |
| water vapour | invisible gas state of water, generally part of the mix of gasses in atmosperic air |