Lights and buoys

Aids to navigation

Navigational aids, “navaid” – also known as aid to navigations, “AtoN” – are special structures such as lighthouses, lightships, beacons, buoys, that are used to enhance safety by providing more opportunities to obtain LOPs.
In contrast: navigational tools including parallel rules, dividers, GPS / Galileo / GLONASS plotters, Radar, compasses, paper charts and sextants.

Shapes

The standard buoy shapes are cylindrical (can) , conical (nun) , spherical , pillar and spar , but variations occur, e.g. barrel .

Beacons – which are mostly fixed and not floating – have only one elongated and upright shape. The shape of the topmark is essential.

Light identification

During daytime, the identification of aids to navigation is accomplished by observing: location, shape, colour scheme, auxiliary features (sound signals, RACON) or markings (name, number, etc).

Yet, during the night, we use the features of the AtoN's light to both identify it and ascertain its purpose. There are three features to describe the light:

• Colour: either W white, R red, G green, Y yellow, Or Orange or Bu blue.
  If no colour is stated in the chart, the default is white.
  On regular charts a white, red, yellow or green lights will be indicated by , whereas on displays and modern multi-coloured charts in specific colours, such as , with the yellow coloured lobe indicating a white light, or if labelled Y, a yellow light.
• Period: the time in seconds needed for one complete cycle of changes.
  
  For example the marker indicates the 10 second period of the two flashes including 8 seconds of darkness of a “Fl (2) 10s” light.
  Except for notably FL (5) 20s , Q (9) 15s and Q (6) + LFL 15s practically all lights have a period of 3 – 10 seconds.
• Phase characteristic: the particular pattern of changes within one complete cycle (hence, within one period). Intrinsically, the faster the changes, the more important (or greater) the danger that is covered by the light.

LFL 10s

Q(3) G 9s

Iso 6s

F Fl Y 5s

Oc 5s

Al WR 4s

Mo (U) 8s   “You are steering a dangerous course”, e.g. near offshore installations.

List of Lights

All lighted aids to navigation are either major or minor lights, where major lights are used for key navigational points along sea-coasts, channels and harbour and river entrances: nominal range over 10 NM.

When not included in the chart, important details, such as colour, structure, pedestals, range, elevation, of these major lights can be found in a vital reference called Light List or List of Lights.

Examples of such “Light Lists” are NGA (world), USA, France (world), Canada, Admiralty (world), Denmark & Greenland, Norway, Iceland, Brasil, New Zealand, etc.

Minor lights are found within harbours, along channels and rivers, and these are of low to moderate intensity: nominal range less than 10 NM.
The range of lights on buoys are never indicated on the chart (superbuoys are the exception) or in a light list.
On a clear night – and without background lighting – the maximum range that a lit mark will be seen is about 3 NM, but often less than 1 NM.
Small harbour lights will have a nominal range of 1 – 3 NM.

Below, an excerpt from the PDF  showing the IALA buoyage system:

Mark, buoy, beacon

Strictly, the term “mark” encompasses both beacons ( fixed ) and buoys ( floated ) AtoNs. The latter are far more prevalent; often you can read the term “buoy” as buoy / beacon.

Region B

The following non-lateral marks are all identical in both the A and B Regions.  

New danger mark

After the sinking of the “Tricolor” in the Pas de Calais (Dover Straits) in 2002, several other vessels hit the wreck despite standard radio warnings, three guard ships and a lighted buoy.

This incident spawned a new type of buoy, the emergency wreck marking buoy, which is placed as close as possible to a new dangerous wreck.

The emergency wreck marking buoy will remain in position until:

• the wreck is well known and has been promulgated in nautical publications;
• the wreck has been fully surveyed and exact details such as position and least depth above the wreck are known; and
• a permanent form of marking of the wreck has been carried out.

Special mark

Marks used to indicate a special area or feature whose nature may be apparent from reference to a chart or other nautical publication. They are not generally intended to mark channels or obstructions where other marks are more suitable.

Body:  yellow, any shape not conflicting with lateral marks.

Topmark:  single yellow " × ” shape

Light:  yellow, for instance Fl Y or Fl(4) Y but any rhythm other than those used for white lights on cardinal, isolated danger or safe water marks.

Used for ODAS, cables / pipelines, recreation zones, boundaries (of anchorages), military exercise zones, renewable energy installations and aquaculture.

Ranges – leading lines

Leading lights and ranges

A range consists of two or more fixed navigation marks situated some distance apart and at different elevations. If lighted these are conventionally called leading lights since “range” applies to distances as well (dipping range, luminous range, geographic range, etc.).

The shape and colours of the daymarks and the colours and characters of lights are advertised in the “List of Lights".

The international standards for describing reduced visibility in marine forecasts are as follows:
Very Poor:  <0.5 NM
Poor:   0.5 – 2 NM
Moderate:   2 – 5 NM
Good:   >5 NM

• Nominal range – the luminous range when the daytime meteorological visibility is 10 NM, which is 74% transmission. Nominal range is generally the figure used in official documentation such as nautical charts, lists of lights, etc. Nominal range assumes that the light is observed against a dark background, free of background lighting.
  If not stated in the chart, consult the list of lights.

The luminous range is the effective nominal range when the atmospheric visibility is not 10 NM.

• The geographic range is based on the elevation of the light, the height of eye of the observer and the curvature of the earth.
  A higher light means that its horizon is farther away, see distance of horizondistance of horizon.
  Similarly, if the observer's height of eye is higher than sea level the light can been seen beyond that geographic range: the dipping rangedipping range.

So, a light – perched on a 70m high cliff – with a geographic range of 20 NM will not be detectable by the human eye (eye height 2m) at a distance of 6 NM, if…

1. …the nominal range is just 5 NM, since the light is not very bright.
2. …the luminous range is just 5 NM, due to a “Light haze”.

Luminous range diagram

This diagram gives the approximate range at which a light may be sighted, at night, in the meteorological visibility prevailing at the time of observation

The diagram is entered from the top border, using the nominal range stated in the nautical chart or the “list of lights”.

The numbers along the curves represent the estimated meteorological visibility at the time of observation, and those along the left-hand border the luminous range under those conditions.

Example: consider a light of an intensity of 100 000 candelas, which equals a nominal range of about 20 NM.
When the meteorological visibility is 20 NM, the light could be sighted at about 32 miles (navy-coloured line, 1 arrow), and when 2 NM, at about 6 NM (navy-coloured line, 2 arrows); given a sufficient elevation and height of eye.

As the scale along the top border is based on a meteorological visibility of 10 NM, the luminous ranges in the prevailing conditions obtained from the bold 10 miles curve will be identical to the nominal range started from the top border.

The diagram can also be used to obtain an approximate meteorological visibility; when, for example, a light of an intensity of 100 000 candelas is sighted at 12 NM, the current meteorological visibility will be about 5 NM.

Caution When using this diagram keep in mind that:

• The ranges obtained are approximate.
• The transparency of the atmosphere is not necessarily consistent between the observer and the light.
• Glare from background lighting will reduce considerably the range at which lights are sighted. A light of 100 000 candelas has a nominal range of 20 NM; with minor background light as from a populated coastline this range will be reduced to about 14 NM, and with major background lighting as from a city or from harbour installations to about 9 NM.

Bottom border: candelas Approximate sighting ranges may be obtained by entering the diagram with the listed intensity divided by 10 for minor background lighting, and by 100 for major background lighting.

Geographic range

A light’s geographic range depends upon the heights of both the light and the observer.

The sum of the observer’s distance to the visible horizon (based on his height of eye) plus the light’s distance to the horizon (based on its elevation) is its geographic range, which is the dipping rangedipping range.

Geographic range = 2.08 × (√Elevation + √Eye height)

For this purpose the formula can be simplified, and solved without a calculator. Assume an opportune standard height of eye of 4 metres as well as rounding 2.08 down to 2.

2 × (√Elevation + 2)

Example with a light elevation of 25 metres:

2 × (5 + 2) = 14 NM

Download the geographic range table (PDF) or geographic range table (PNG) or use my online calculator.

See the geographic range table or use my online calculator.

Visible range

When comparing the geographic range with the light’s luminous range, then the lesser of the two ranges is the range at which the light will first be sighted: the visible range.

Plot a visibility arc centered on the light and with a radius equal to the visible range. Extend the vessel’s dead reckoningdead reckoning track until it intersects the visibility arc.
The bearing from the intersection point to the light is the light’s predicted bearing at first sighting.

Bobbing a light

When first sighting a light, an observer can determine if it is on the horizon by immediately reducing his height of eye. If the light disappears and then reappears when the observer returns to his original height, the light is on the horizon. This process is called “bobbing a light”.

Tide or no Tide…

There will be some judgement involved (luminous range is a rough estimate) resulting in a large margin of error in visible range. Therefore only when light elevation is rather low (<20m), while tidal range is high should it be necessary to include tide.

Loom

Because of the limiting factor of the geographic range, most major lights will never be seen from a sailing yacht 20 NM away. Yet, due to atmospheric scattering, it is sometimes possible to take a compass bearing on the loom of the light: its reflection against the clouds. Additionally, it is sometimes even possible to observe a rotating beam of light.

Loom	Dipping distance / range

Colours have different ranges

Different coloured lights with equal luminous intensity have different ranges.
White light is the most visible followed by yellow, green and then red.
Therefore, at extreme ranges an “AL WG” can resemble a “Fl W”.  

Distance of minor light

The range of a lighted buoy is never indicated, but on a clear night the maximum range is 3 NM, yet often considerably less.

There are 2 visual indicators to determine your distance from a buoy: at about 0.5 NM, the light will rise up from the horizon, and at about 200m, the light will reflect in the surface.

Buoy at less than 3 NM	Buoy at less than 0.5 NM	Buoy at less than 200m

Glossary

• Loom: the diffused glow observed from a light below the horizon, due to atmospheric scattering of its light rays, usually the undersides of clouds.
• Looming: an apparent elevation of distant terrestial objects by abnormal atmospheric refraction. Because of looming, objects below the horizon are sometimes visible. The opposite is “sinking” during low visibility.
• ENC: the Electronic Navigation Chart.
• ECDIS: the Electronic Chart Display and Information System.
• AIS Aids to Navigation: Automatic Identification System AtoN
  • Real AIS AtoN is attached to a physical mark such as a lighthouse or buoy.
  • Synthetic AIS AtoN is also associated with a physical mark, but the actual AIS transmitter is in another location.
  • Virtual AIS AtoN marks the hazard with the hazard’s coordinates, but there is no physical light, buoy.
• ODAS buoy: Ocean Data Acquisition Systems buoy – means a buoy intended for the collection of data on properties of the ocean. It may be moored or free-floating.
• Aid to Navigation (AtoN) or Navigational aid (NAVAID): any mark, sign, device or system external to vessels that is designed and operated to assist in determination of position, to define a safe course, or to warn of dangers, vessel traffic or obstructions.
• Nominal range is the luminous range when the meteorological visibility is 10 NM, equivalent to a transmission factor of T=0.74.
• Geographical range is the maximum distance at which an object or light from a light source can theoretically be seen by an observer, as limited only by the curvature of the earth, the refraction of the atmosphere, the elevation of the object or light and the height of the observer’s eye.
• Luminous range is the maximum distance at which a given signal light can be seen by the eye of the observer at a given time, as determined by the intensity of the meteorological visibility prevailing at that time. It takes no account of elevation, observer’s height of eye or the curvature of the earth.
  Example – a light of an intensity of 500 candelas (nominal range of approx. 8 NM), can be seen up to 12 NM when the meteorological visibility is 20 NM, but will be seen only at 3 NM if the meteorological visibility is 2 NM.
• Visible range: the extreme distance at which an object of light can be seen, with current height of eye and current meteorological visibility: the lesser of the luminous and geographic ranges.
• Bobbing a light: quickly lowering the height of eye and raising it again when a navigational light is first sighted to determine if the observer is at the geographic range of the light.
• Luminous range diagram: a diagram used to convert the nominal range of a light to its luminous range under existing conditions.
• Meteorological Optical Codes: ranks from 0 (dense fog : less than 50 metres of visibility) to 9 (exceptionally clear : more than 27 kilometres of visibility).
• Mark, seamark, navigation mark: an artificial or natural object of easily recognizable shape or colour, or both, situated in such a position that it may be identified on a chart. A fixed artificial navigation mark is often called a beacon.
• Light characteristics: the sequence and length of light and dark periods and the colour or colours by which a navigational light is identified.
• Topmark: one of more objects of characteristic shape placed on top of a buoy or beacon to aid in its identification.
• Lateral mark: AtoN intended to mark the sides of a channel or waterway.
• Cardinal marks: black and yellow AtoN intended to show the location of a danger to navigation based on its position relative to the danger using the cardinal points of the compass: North, East, South and West, each with distinctive double-cone topmark variations. If lighted, the Q or VQ group flashes resemble the face of a clock.
• Isolated danger Mark: AtoN marking a danger with clear water all around it; it has a double ball topmark and is black with at least one red band. If lighted its characteristic is Fl(2).
• Sector light: a fixed AtoN that displays a light of different colours and / or rhythms over designated arcs. The colour of the light provides directional information.
• Light sector: as defined by bearings from seaward, the sector in which a navigational light is visible or in which it has a distinctive colour difference from that of adjoining sectors, or in which it is obscured.
• Lighthouse: a tower, or substantial building or structure, erected at a designated geographical location to carry a signal light and provides a significant daymark. It provides a long or medium range light for identification by night.
• Minor light: a light that has nominal range <10 NM. An automatic unmanned light on a fixed structure usually showing low to moderate intensity. Minor lights are established in harbours, along channels, along rivers, and in isolated dangers.
• Major light: a light that has nominal range >10 NM. A light of high intensity and reliability exhibited from a fixed structure (lighthouse) or on marine site (except leading lights). Major lights include primary sea-coast and secondary lights.
• Light List: a detailed list of navigational aids including lighthouses and other lighted navigation aids, unlighted buoys, radiobeacons, daybeacons and racons.
• Landfall: the first sighting, by eye or by radar, of land when approached from seaward.
• Range: two or more objects (leading marks) in line. Such objects are said to be in range. An observer having them in range is said to be “on range line”, or “on track”. Two beacons are frequently positioned together for the specific purpose of forming a range to indicate a safe route or the centerline of a channel.
• Leading line: a straight solid line, drawn through leading marks (a range) on the chart. A ship moving along such line will clear certain dangers or remain in the best channel.
• Leading lights: two or more lights at different elevations so situated to form a range and its leading line when brought into transit. The one nearest to the observer is the from light and the one farthest from the observer is the rear light. The front light is at a lower elevation than the rear light.
• Lights in line: two or more lights so situated that when observed in transit they define a position: the limit of an area, an alignment used for anchoring, etc. Not to be confused with leading lights, which mark a recommended direction to be followed.