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Glossary

Navigation glossary

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

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Tides glossary

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)

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Charts glossary

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

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COLREG glossary

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)

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Weather glossary

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
 
 
 

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