Navigation Study guide - Squarespace · Navigation – Study guide ... Passage Planning ... It is best practice to use the latitude scale at the same latitude as the plotted position

Navigation – Version 1 (2013) ©Coastguard New Zealand 2014

1

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


2

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


3

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

http://www.linz.govt.nz/


4

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, south-

east, 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.


5

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).

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.

True course 150°T

Variation 18°E

Magnetic course 150°

- 18°

132°M


6

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:

The compass course to steer is therefore 253°C.

True course 270°T

Variation 021°E

Magnetic course 270°

-021°

= 249°M

Deviation 004°W

Compass Course 249°

+004°

253°C


7

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.


8

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.


9

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


10

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.


11

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.


12

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.


13

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 the 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.


14

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.


15

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.


16

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 method;

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


17

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

Documents

Navigation Study guide - Squarespace · Navigation – Study guide ... Passage Planning ... It is best practice to use the latitude scale at the same latitude as the plotted position