Navigation – Study guide
Contents
Overview................................................................................................................................ 2
1. Navigation and Charts ....................................................................................................... 2
1.1 Chart Symbols ............................................................................................................. 2
1.2 Chart Measurements and Plotting ............................................................................. 2
1.3 How to Correct a Nautical Chart.................................................................................. 3
2. The Compass...................................................................................................................... 4
2.1 Magnetic Variation....................................................................................................... 4
2.2 Deviation ...................................................................................................................... 5
2.3 Application of Variation and Deviation ........................................................................ 6
3. The Tide ............................................................................................................................. 7
3.1 Tidal Heights................................................................................................................. 7
3.2 Tidal Streams................................................................................................................ 7
3.3 Tidal Stream Tables ...................................................................................................... 8
3.4 Currents........................................................................................................................ 8
4. Confirming Position........................................................................................................... 9
4.1 Transits......................................................................................................................... 9
4.2 Position fix.................................................................................................................. 10
4.3 Depth.......................................................................................................................... 11
4.4 Plotting Radar Bearings.............................................................................................. 11
4.5 Plotting Radar Ranges ................................................................................................ 12
4.6 Dead Reckoning and Estimated Position ................................................................... 12
4.7 Running Fixes ............................................................................................................. 13
4.8 Clearing Lines and Marks ........................................................................................... 14
5. Passage Planning ............................................................................................................. 15
5.1 Calculating the Time of the Passage .......................................................................... 15
5.2 Planning...................................................................................................................... 15
5.3 Contingency Plans ...................................................................................................... 17
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Overview
Coastguard personnel need to develop familiarity with navigation on board a CRV with full, partial or none of
the usually available navigation aids. Skills required include plotting reported positions on paper charts and the
vessel’s chart plotter, using a hand-bearing compass, transits and depth to confirm the CRV’s position, using
Radar to fix a position on a paper chart and to establish a safe distance off any hazards, and constructing a
course to steer from a paper chart and from the vessel’s chart plotter.
1. Navigation and Charts
The chart title block is the first thing that is used to evaluate the suitability of a chart as a navigation tool for
use in a particular area.
Elements of the Title Block are:
 the name of the geographical area that is depicted on the chart
 the unit of measure used to express depth
 the scale. The largest scale chart available for the area should always be used.
 details of how depths and heights are expressed in relation to chart datum
 the IALA Maritime Bouyage System Region. New Zealand is Region A.
 the basis for positions. WGS 84 is the basis used for the global positioning system
 other information specific to the charted area and the chart detail
1.1 Chart Symbols
The information on charts is given using various symbols and abbreviations.
Chart 5011 (INT 1) Symbols and Abbreviations used on Admiralty Charts, Edition 3, published by the United
Kingdom Hydrographic Office gives an explanation of all the symbols, terms and abbreviations used in New
Zealand charts, and is available for purchase online.
1.2 Chart Measurements and Plotting
The latitude scale on the sides of the chart when orientated North up is used to measure distance (nautical
miles).
1 minute of latitude = 1 nautical mile
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It is best practice to use the latitude scale at the same latitude as the plotted position in order to measure
distance. This is due to the spacing of the parallels of latitude (and thus the length of a minute) increasing with
their distance from the equator, due to the shape of the Earth.
A position is plotted on a chart in terms of the latitude and longitude scales. When plotting a course, specific
symbols should be used to avoid confusion.
Fix from Waypoint
Fix from Bearings
Fix from Radar Ranges
Transferred Position Line
Ground track
Estimated Position
Dead Reckoing Fix
Water Track
Tidal Vector
1.3 How to Correct a Nautical Chart
Corrections and updates to charts are necessary where changes occur that will affect navigation.
For safety reasons it is extremely important to keep charts updated. In New Zealand, fortnightly notices (New
Zealand Notices to Mariners) are published by government agency LINZ (Land Information New Zealand) on
the internet, and can also be sent out by email if requested (see LINZ website: http://www.linz.govt.nz).
Mariners are also advised to regularly check broadcast warnings by monitoring VHF Channel 16.
To correct a chart:
1. Obtain the latest corrections from LINZ for the local area
charts.
2. Make the correction
Use a magenta coloured pen for permanent corrections and a pencil
for temporary corrections.
If replacing a symbol with another, draw the new symbol as close to
the old one as possible, taking care not to cover any other
information; then draw a curved line linking the new symbol to the
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correct position. Finally, cross out the old symbol by drawing two
diagonal lines through it.
If inserting a new symbol, draw it in the exact position stated. If
there is no room to do this, draw it as close as possible to the
required position and draw a curved line linking it to the exact
position. Finally, note the details of the correction, including the
date and number of the Notice, in the bottom left hand corner of
the chart.
Electronic charts need to be kept up to date. Some can be easily corrected online, while others require a
replacement memory card from the manufacturer.
2. The Compass
A magnetic compass contains a magnet that aligns itself with the magnetic field of the earth, thus indicating
the direction of magnetic north. The compass card is the compass face that gives gradations in degrees (0° –
360°) (usually along with the cardinal and inter-cardinal points of north, east, south, west, north-east, southeast, and so on). It is attached to one or more magnetic bars and mounted on a pivot so that it can turn freely.
In addition, in a marine compass, the compass card is generally suspended in a liquid that reduces the amount
of movement caused by a boat.
2.1 Magnetic Variation
Variation refers to the difference between Magnetic North and True North. Magnetic North is the relative
direction of the Earth’s north magnetic pole which changes slightly from year to year. The chart is always
orientated True North up.
The angle of variation changes relative to position on the Earths surface. Variation ranges from around 30
degrees east to 30 degrees west of True North.
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For example, in New Zealand, magnetic variation will range from 18° to 25° east of True North. Magnetic
variation also changes slightly over time with the movement of the earth’s plates.
The variation for different areas, along with the annual rate of change, is given inside the compass rose on the
relevant charts.
Calculating variation
To convert a course from Magnetic North to True North
Add easterly variation
To convert a course from True North to Magnetic North
Subtract easterly variation
To the west of the Prime Meridian (line of 0° longitude) the magnetic reading is greater than the true reading
‘Variation West – Magnetic Best’.
To the east of the Prime Meridian the magnetic reading is less than the true reading ‘Variation East – Magnetic
Least’. This will always be the case in New Zealand. Another way of remembering this is to use the acronym
CADET (From compass to true, add east).
For example, if the true course to a waypoint is 150°T, and the variation is 18°E, then the magnetic course to
steer is 132°M.
Use T or M after the reading to differentiate between True and Magnetic North (e.g. 200°M; 220°T).
True course
150°T
Variation
18°E
Magnetic course
150°
- 18°
132°M
2.2 Deviation
Deviation is the error in a particular vessels compass reading caused by magnetic fields on a vessel.
These are generated by steel and iron in the vessel’s hull, high current electrical cables, and items
such as radios and speakers. Even if a CRV hull is constructed of aluminium, which has no effect on
the compass, it will usually have electronics and radios in the proximity of the compass that may
have an effect.
Tools or portable electronic devices ought not to be placed in close proximity to a compass as they
may have a significant impact on the accuracy of the compass and therefore the safety of the vessel.
A hand-held compass should be used well clear of these objects. If using a mounted steering
compass, this may have been corrected to eliminate deviation, or the deviation card for that
compass listing the deviations for the various different headings must be used for calculations.
A vessel’s magnetic field will change over time,or with the installation of new electronics, therefore
the values on a deviation card should be periodically checked against the bearing of a charted
transit. Coastguard CRV’s are commercial vessels and therefore must have the compass swung and
the deviation card updated every four years.
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2.3 Application of Variation and Deviation
On a trip along the East coast of the South Island of New
Zealand. If the course is 270°T. The variation is 021°E and the
deviation on this course is 004°W (deviation at 270°T as
shown on the compass card).
Then the calculation is:
True course
Variation
Magnetic course
Deviation
Compass Course
The compass course to steer is therefore 251°C.
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270°T
021°E
270°
-021°
= 249°M
002°W
249°
+002°
251°C
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3. The Tide
Ocean tides are caused by the gravitational influence of the sun and moon. When the sun, earth and moon are
in alignment, the gravitational pull is stronger, and the difference between low and high tides is greater. This is
known as a spring tide. When the sun, earth and moon are at right angles, the gravitational pull is weaker, and
the difference between low and high tides is less. This is known as a neap tide.
The tide is further influenced by the effects of weather pressure systems, wind, the geography of the coastline
and seafloor, and the tilt of the earth’s axis.
3.1 Tidal Heights
Local tide tables enable you to
determine the times and heights of
tides at different places around the
coast. Tidal heights are given above
Chart Datum – the lowest tide level to
be expected in average weather
conditions. The information from tide
tables can be quickly plotted on a
graph to find out the depth of the
water at any time between low and
high tide.
Alternatively, the information can be
plotted on a tidal curve. The
information in tide tables in the
Nautical Almanac is usually given for
standard ports only, so the information for secondary ports will need to be calculated. Secondary ports are
minor ports that are often associated with a standard port with similar tidal patterns. The information
required to calculate the times and heights of tides at secondary ports is given in the Nautical Almanac.
It is important to know the tidal information that is specific to the place in which, and specific to the time at
which the CRV is navigating. As well as being able to establish whether there will be sufficient water to avert
running aground, the tidal height and time relative to high and low water can be used in conjunction with
weather data, to give valuable information to the navigator as to the expected sea state.
3.2 Tidal Streams
A tidal stream is a periodic movement of water in a horizontal
direction that is due ultimately to the same astronomical causes
as the tide. The tide is a movement in the vertical direction.
The tidal stream during a rising tide is known as a flood stream;
during the falling tide, it is known as an ebb stream. There is
usually a period when the flow eases, known as slack water,
before the stream changes direction.
On a chart, the direction of flow of a flood stream may be shown
by an arrow with feathers on its tail; the direction of an ebb
stream is shown by a plain arrow.
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3.3 Tidal Stream Tables
The direction (set) and speed (rate) of a tidal stream can be predicted for areas in which sufficient observation
and recording of data has been carried out. This information is given in tables on charts. On a chart, tidal
reference points are given in specific locations as a capital letter inside diamond shape. A table at the edge of
the chart gives information on the set and rate of the tidal stream at these locations at different times. Tidal
stream rates are faster during spring tides and weaker during neap tides.



Set is measured on the chart in degrees true.
Rate is measured in knots. On the chart, the rate given next to the directional arrows is for a spring
tide. If two rates are given, these are for both spring and neap tides.
Drift is the distance the water moves (in nautical miles) over a given period of time, and is obtained
by multiplying the time in hours by the rate.
Tidal time and rate predictions are estimates only, and weather patterns can affect the tidal stream. All
available tidal and weather information should be combined with other sources of local knowledge in order for
the navigator and crew to build up a realistic picture of the prevailing conditions, prior to the CRV departure.
3.4 Currents
A current is a strong flow of water in one direction. Surface currents are usually caused by wind, deeper
currents are caused by factors such as temperature or density differences in water and geography. Currents
are indicated on a chart by an arrow with a rippled line and rate of flow given in knots.
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4. Confirming Position
Always use all available means to verify information from a navigational aid such as a chart plotter to establish
position. As with all electronic devices, the chart plotter can fail at times. A good navigator will always use all
available means to verify the vessels position. It is good practice to obtain regular position fixes using a
compass and/or radar ranges in combination with other methods (e.g. transits, contour lines, depth soundings)
in order to keep track of the vessel’s progress.
GPS does not allow for tidal drift or may not indicate water depth, and doesn’t show whether or not it is safe
to follow a particular course. This is particularly relevant for a CRV if returning to a preselected course having
stopped for a length of time and potentially drifted.
4.1 Transits
Two fixed reference points that are directly in the same line of sight are said to be in transit. This can be drawn
on a chart to help determine a vessel’s position. The vessel’s position will lie at some point along this line. One
way to obtain a position line is to take a compass bearing of a visible charted object, convert this to a true
bearing and then plot it on the chart.
Transits can be natural features, or systems of navigational beacons that have been set up in and around ports.
These beacons are arranged in pairs in such a way that when they line up, they form a position line which a
vessel should follow to safely leave or enter the port.
Three examples of transits
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The Beacons, known as ‘leads’, are identified by their distinctive topmarks in the day, and distinctive lights (e.g.
fixed or flashing pattern; white, coloured, or alternating colours) at night (when they are known as ‘leading
lights’). The boundaries between different-coloured sector lights are marked on a chart with dotted lines. If the
vessel crosses one of these boundaries, there will be a change of colour of the lights observed. This border
between light sectors is a very good position line that can be plotted on the chart and used to obtain a position
fix.
The leads will be marked on the chart, and listed in the Almanac, as well as in the ‘Admiralty List of Lights and
Fog Signals’ (produced by the UK Hydrographic Office).
4.2 Position fix
In order to obtain a position fix the bearings of two (or more) visible features that are shown on the chart must
be taken and plotted. The point where the lines intersect gives the vessel’s position. Plotting a third position
line will give a more accurate fix.
If when plotted the three lines cross in a triangle shape – known as a ‘cocked hat’ – rather than intersecting at
the exact same point there is some degree of error in the fix. The larger the triangle, the greater the error. If
the ‘cocked hat’ is too big the fix should be retaken, perhaps using different features, and replotted.
Important points to remember when using this method:
 It is better to take bearings of land-based features rather than buoys, as buoys are likely to move with
the rise and fall of the tide or in bad weather.
 The chosen features should not be too close together, as this will reduce the accuracy of the fix. The
angle at which the first two position lines intersect (the ‘angle of cut’) should ideally be 60° – 90°, and
in the case of a third line, there should be an arc of no more than 120° between the outermost
position lines.
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
Errors in a position fix can be caused by a variety of factors, including calculation error, plotting error
and compass error. If there are any hazards in the area, it is safer to assume that the vessel is closer
to these, rather than further away.
Transit bearings have several advantages over compass bearings, namely:
 They are quicker and easier to establish as the method simply involves identifying the two objects on
the chart.
 They are more accurate, as the features are in a fixed position, and there are no errors caused by
factors such as variation and deviation.
 A position fix can be obtained by crossing the transit bearing position line with one or two
(preferable) compass bearing position lines.
4.3 Depth
Sounding the depth of the water using an echo sounder and comparing this to the information on the chart
can help to improve the accuracy of a position fix. A contour line can be used as a position line provided that:
 The present height of the tide is taken into account.
 The depth of the echo sounder’s transducer below the water is factored in (unless this is done
automatically by the echo sounder).
4.4 Plotting Radar Bearings
Radar can be used to detect the bearing of a target relative to the vessel, and the range, or distance, of the
target from the vessel. For ease of plotting bearings, ideally use the radar in north up mode.
The Electronic Bearing Line (EBL) is used to determine the relative bearing of the target. Record the vessel
heading at the exact time the bearing using the EBL is taken.
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To calculate the radar bearing add or subtract the EBL bearing to or from the heading bearing. Then convert
the radar bearing to a true bearing using variation and deviation. Plot the true bearing on the chart.
4.5 Plotting Radar Ranges
The Variable Range Marker (VRM) is used to determine the distance (range) to a target. A position circle (or
arc) can then be marked on the chart using a pencil compass.
The position circle can then be crossed with one or two other position circles or position lines to obtain a
position fix. If just one position line is used, it will cross the circle at two points, and it may or may not be clear
at which of these points the vessel sits. Plotting a third position line (or circle) should remove this ambiguity.
4.6 Dead Reckoning and Estimated Position
If visibility is poor or the vessel is a long way from landmarks it may not be possible to use any charted features
to plot position lines. In such cases, two methods can be used to estimate the vessels position. These are ‘dead
reckoning’ (DR) and ‘estimated position’(EP).
Dead reckoning involves drawing an extrapolated line on
the chart bearing the same as the vessels true course. To do
this the start point must be known, along with the course
that has been steered and the distance travelled. For this
reason it is extremely important for the navigator to
regularly record the vessel’s position.
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By drawing an extrapolated line corresponding to the vessels true course on the chart, the DR position can be
obtained. The water track (distance run and course steered) can be shown by a line with a single arrowhead in
the middle; the DR position can be shown by a small line crossing the water track, and the letters ‘DR’ written
next to it. The time must also be recorded. Dead reckoning does not take into account the tide or wind.
An EP gives a better indication of actual position, as it takes into account the effects of tide and leeway (where
relevant). To work out the estimated position, the direction of the tide (set) and tidal drift must be known. In a
coastal area, the direction and speed of the tidal stream may be given by the tidal diamond table on the chart.
To mark the estimated position on the chart plot the DR position, then draw the tidal vector at th e end of the
water track. The tidal vector is shown by a line with three arrowheads in the middle, and the EP is shown by a
triangle on the position at the end of the tidal vector. The ground track (the actual track the vessel would take
from the start point to the EP, if viewed from above) is plotted as a line with two arrowheads in the middle.
Once the EP is marked the time is recorded on the chart beside it. The EP should be plotted regularly and at
every change of course in more confined waters. Allow for the corresponding period of the tide.
When considering the reliability of the different methods for fixing position, the Fix is the most reliable,
followed by Running fix, Estimated position and the least reliable is Dead reckoning position.
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4.7 Running Fixes
In cases where there is only one visible charted feature a ‘running fix’ (or ‘transferred position line’) will need
to be taken. This involves taking two separate bearings of the one feature at two separate points in time.
In order to calculate a running fix:
1. Take the compass course and convert to True. Log the vessel speed.
2. Take a bearing off the visible object and plot a position line on the chart. Note the time.
3. A period of time later take a second bearing off the same object. Plot this and record the time.
4. Plot the water track starting from any point along the first position line to show the direction and
distance travelled.
5. From the end of the water track, draw the tidal vector to show the distance and direction the tide has
carried the vessel.
6. Plot a position line (the ‘transferred position line’) that is parallel to the first position line and crosses
the end of the tidal vector. Put a double arrowhead at the end to show that this is a transferred
position line.
7. The running fix is the point where the transferred position line intersects with the second position
line. Draw a circle around this point to show the running fix.
4.8 Clearing Lines and Marks
In order to ensure that the vessel is clear of an identified danger (e.g. hidden rocks at the entrance to a
harbour), a clearing bearing can be plotted from a clearly identifiable charted object. As long as the vessel
passes to, or remains on a certain side of this bearing line, it will be clear of the danger. The bearing can then
be measured using a compass, and used to navigate clear of the danger.
Depth can also be used as a clearing line, taking into account the tide at the time. For example, on a particular
section of a route, it may be that the vessel is clear of all dangers at a depth of 20 metres or more, so the
water depth should be carefully monitored, and the shallow depth alarm set on the depth sounder. Transit
bearings can be used as clearing marks to show when a boat is clear of danger.
These may be marked on the chart or appropriate charted landmarks can be selected. When these two
features are seen in transit, this should indicate that the vessel is safely clear of the danger.
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5. Passage Planning
5.1 Calculating the Time of the Passage
The time the passage will take is calculated by dividing the distance to be travelled by the vessel’s estimated
speed. To carry out the calculation work with either water speed or ground speed (SOG) and the
corresponding distance (i.e. water track distance or ground track distance).
Time of passage = water track distance / speed through water
OR
Time of passage = ground track distance / speed over ground
A vessel travelling at 6kn will take 10 mins to travel 1nm. This information can be used to perform a
quick logic check of calculated times and distances, or to keep calculations simple by using multiples
of 6kn which divide equally into an hour. All calculations should be properly checked and confirmed
before being passed to the helm.
5.2 Planning
Many accidents at sea are caused by the misuse of navigation tools and incorrect interpretation of
information. Good planning and preparation before undertaking a passage can reduce risks. It may be useful to
plan courses from vessel launch point to ‘popular’ local trouble spots in advance.
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This will allow the crew to:
 Identify any likely problems or danger spots
 Ensure correct charts are on the vessel
 Make contingency plans and identify safe havens
 Spend less time en route doing position fixes and planning the course to steer, and more time keeping
a lookout for landmarks and waypoints
 Consider the crew members and their experience and capabilities; it is not a good idea for only one
person (e.g. the skipper) to know what the plan is or how to do all the necessary tasks involved in
navigating the vessel
 Think about the vessel and its equipment and decide if it is suitable for the passage
The following are the steps to follow when planning and executing a passage:
Before Departure
 Ensure tidal heights, tidal streams, currents, navigational lights, navigational marks, any navigational
warnings from the Notices to Mariners, and the latest weather reports have been taken into account
 Circle any dangers on the proposed route (e.g. rocks) and determine safe passing distances, using
clearing marks / lines where necessary
 Ensure the route chosen does not exceed the capabilities of the vessel, or the crew or available
navigational equipment
 Select positions to alter course that correspond with identifiable charted objects
 Calculate and note down the compass bearing for each leg of the course, along with the distances, the
waypoint numbers and the charted features
 Select and enter waypoint positions (to correspond with the alter-course positions) into the GPS
 Ensure that other crew members are fully aware of the passage plan
 Check that the vessel’s navigational equipment is working well
 Know the local radio channels, plan scheduled radio calls – consider who to, when and on which
channel
Executing the Passage
 Keep progress of the passage by taking and recording position fixes at regular intervals using a
compass and/or radar ranges in combination with other methods (e.g. transits, contour lines, depth
soundings)
 If at any stage during the passage the vessels position can not be fixed, stop the vessel until a position
fix can be made; if there is only one visible charted object a running fix may be taken
 If the vessel is found to be off course at any stage, take another position fix using a different me thod;
if the (off-course) fix is confirmed, change the vessel’s heading to bring it back to the intended course
 Constantly monitor the weather by observing the clouds and the wind speed and direction; check
weather forecasts regularly
 Maintain a listening watch on the radio for navigational warnings and other vessel movements that
may impact on the execution of the CRV passage plan
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5.3 Contingency Plans
Things do not always go according to plan, and conditions may change during the passage. It is therefore
advisable to have thought through alternative options and contingency plans well in advance. Specifically:
 If the weather turns bad, safe havens may be needed to seek shelter; study the chart beforehand to
identify suitable anchorages
 Where there is limited visibility (e.g. due to fog or weather conditions), reduce speed, and use
navigation lights and the appropriate sound signals; to fix the vessel’s position, use alternative means
to the three point position fix, such as a running fix, GPS, radar or echo sounder
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