Charts, Datum and

UNIT 10 CHARTS, DATUM AND COMPASS Compass Corrections

CORRECTIONS
Structure
10.1 Introduction
Objectives
10.2 Great Circle, Small Circle, Poles of a Great Circle
10.3 Basic Knowledge of Chart Projections
10.4 Natural Scale of a Chart
10.5 Types of Chart
10.6 Indian and Admiralty Chart Catalogue and Folio System
10.7 Corrections of Charts and Other Publications
10.8 Rotation of the Earth
10.9 Direction on the Surface of the Earth
10.10 Measuring Distance Between Two Points on Mercator Chart
10.11 True, Magnetic and Compass North, Variation and Deviation
10.12 Deviation Card/Table and Variation from Chart
10.13 Finding True Course from Compass Course
10.14 Finding Compass Course from True Course
10.15 Compass Error from Transit Bearing
10.16 Applying Compass Error to Compass Course and Compass Bearing to
Find True Course and True Bearing
10.17 Summary
10.18 Key Words
10.19 Answers to SAQs

10.1 INTRODUCTION
Navigation charts are a systematic representation of the features of land and the sea on
paper so that information required by a navigator can be viewed at a glance.
Early handmade charts date back to the 14th century. Over the last 700 years, however,
they have gone through various stages of development, thanks to the efforts made by
some pioneer cartographers and navigators in the 16th century. Nations of northern
Europe, particularly Holland, are believed to have played an important role in production
of charts for seafarers.
Objectives
After studying this unit, you should be able to
• identify various types of charts,
• acquire knowledge of direction on the earth’s surface,
• differentiate between the compass North, Magnetic North and true north,
• calculate deviation and variation,
• calculate total error,
• apply compass error to compass bearings to obtain true bearings, and
• apply compass error to compass courses to obtain true courses. 5
Practical Navigation
10.2 GREAT CIRCLE, SMALL CIRCLE, POLES OF A
GREAT CIRCLE
Great circle is a circle on the surface of a sphere, the plane of which passes through the
centre of the sphere. For example, Equator is a great circle and all meridians are semi-
great circles.
Small Circle is a circle on the surface of a sphere, the plane of which does not pass
through the centre of the sphere. For example, all parallels of latitudes are small circles.
Poles of a Great Circle are two points on the surface of sphere diametrically opposite to
each other, from where every point on the great circle is 90o away.
In Figure 10.1, AB is a great circle and P & P´ are its poles. CD and EF are small circles.
P

C D

A B

E F

P´

Figure 10.1

10.3 BASIC KNOWLEDGE OF CHART

PROJECTIONS
A projection is a means of representing a spheroidal surface on a plane. It is almost
impossible to fit exactly a spheroidal surface on a plane surface. Thus, earth’s surface
which is shaped like a spheroid, when shown on paper, will have a distortion, no matter
what projection is used.
There are various projections used in making charts. In choosing a particular projection
for constructing a chart, we first decide as to what kind of distortion is least objectionable
and what particular properties are to be fulfilled by the chart.
The chart used for navigation should represent the shape of the land correctly. As the
most common form of sailing is along rhumb line tracks, the navigator wants the rhumb
lines to be represented by straight lines and the distances between two points can be
measured easily. Most of the navigational charts are constructed on Mercator’s
projection, as this projection fulfils the requirements of the navigator.

10.3.1 Mercator’s Projection Charts

Distortion must take place when earth’s surface is represented on chart.
(a) In Mercator’s projection, meridians are represented as straight lines at right
angles to the equator and, thus, divide equator into 360 equal parts.
(b) Equator and parallels of latitude appear as horizontal parallel straight lines at
selected distances from equator and from each other.
6
On earth’s surface, meridians converge towards the pole and the distance between Charts, Datum and
meridians is maximum at equator and reduces as the latitude increases. But on Mercator’s Compass Corrections
chart, meridians are represented by equidistant parallel straight lines. East-west
distortion, therefore, increases as latitude increases.
Thus, distances between successive parallels of Latitude on Mercator’s chart is increased
towards the poles and distortion increases towards poles. Latitude scale at equator is
same as longitude scale but varies as latitude increases.
Lat. scale = Long scale × sec. Lat.
On Mercator’s chart, the longitude scale is constant throughout, as the equator is divided
into 360 equal parts, but latitude scale increases with latitude resulting in an increase in
the length of a nautical mile pole wards.

10.4 NATURAL SCALE OF A CHART

The natural scale of a chart is the ratio that the distance between two points on the chart
bears to the actual distance between them on the earth.
For example a natural scale of 1/ 25,000 means that one unit of length on the chart
represents 25,000 units of length on the earth. In other words, 1 cm on the chart
represents 25,000 cm on the earth, or one foot on the chart represents 25,000 feet. on the
earth. As the latitude scale changes with the latitude, the natural scale of a Mercator chart
varies from latitude to latitude. Therefore, any natural scale stated on the chart is valid for
a particular latitude only.
Natural scale is normally expressed as the relationship that one minute of longitude on
the chart bears to one minute of longitude on the earth, in that latitude.
If one minute of longitude on a chart is represented by 5 mm in latitude 60o, the natural
scale in that latitude can be obtained as follows:
Chart Distance
Natural Scale =
Earth Distance
The chart distance for 1´ of long. = 5 mm
Since one minute of longitude on the earth at the equator is equal to
1 mile = 1852 m = 1852 × 1000 mm , the length of one minute of longitude in latitude 60o
would be the departure in that latitude corresponding to a difference of longitude of
1' i.e. 1 × 1852 × 1000 × cos 60o
5 1 10 1
Natural scale = ÷ 18,52,000 × = =
1 2 18,52,000 1,85, 200
From the foregoing, it will be realized that a Mercator chart of any area can be
constructed quite accurately to a given natural scale in a particular latitude.

10.5 TYPES OF CHART

Nautical charts can be classified into three categories
(a) Ocean Charts
(b) Coastal Charts
(c) Plan Charts
10.5.1 Ocean Charts or Small-Scale Charts
They are small-scale charts covering a large part of the earth’s surface. These are used
mainly on trans-ocean or trans-sea voyages. They are unsuitable for coastal navigation or
for plotting positions with small time intervals between them as the positions quite often 7
Practical Navigation tend to overlap one another due to the small scale of the chart. The scale of these charts
ranges from 1 : 600,000 to 1 : 14,000,000. The ocean charts show little detail apart from
the sounding, the positions of radio beacons and lights and other relevant information,
which may be useful on an ocean passage.
10.5.2 Coastal Charts or Medium-Scale Charts
Medium-scale charts are used for coastal navigation. These are also divided into two
subcategories depending on the scale of the chart:
• Natural Scale of charts used for coasting ranges from 1 : 150,000 to
1 : 600,000 and such charts are often called general coastal charts.
• Natural scale of the inshore charts, however, ranges from 1 : 50,000 to
1 : 150,000. Such larger scale charts are known as inshore charts.
The general charts are mainly for coasting and have the major navigational aids and
details marked on them. The inshore charts, being larger in scale, are in greater detail and
quite often used as approach charts to ports and harbours. A prudent navigator should
always use the largest scale chart of an area that is available.
10.5.3 Plan Charts or Harbour Charts
They are large-scale charts of small areas such as ports and harbours. Quite often, they
are called harbour charts. Their scale is usually larger than 1 : 50,000 and can range up to
1 : 2,500. These charts display data of harbours, ports, approaches, berths, jetties, rivers
and canals etc. These charts contain detailed information required to navigate a ship in
narrow congested areas.
Apart from the charts mentioned so far, there are different types of charts for different
uses.
Lattice Charts
These are nautical charts, which have a lattice pattern of a particular electronic
navigation system superimposed upon them. Among the lattice charts in use today
are charts for Decca, Loran C. With the introduction of the latest receiver sets for
each of these systems which display the ships position in latitude and longitude,
the use of the lattice charts has further diminished. They are, however, used by
vessels which have receivers of the older models.
Routeing Charts
These are informational charts. These charts are published every month of the year
for different oceans and they have recommended ocean passages printed upon
them. These charts also show the predominant currents, wind patterns, occurrence
of reduced visibility, sea water temperature, relative humidity, air temperatures,
isobars and ice conditions for the particular month in that area of the ocean.
Variation Charts
The British Admiralty issues a special chart, which shows Isogonic lines. Isogonic
lines are lines joining points of equal magnetic variations. This chart is particularly
important on board vessels which are without a Gyro compass but should be
available on board every ship. The variation on this chart is more accurate than
those shown on the compass rose of many charts. Variation chart is updated every
few years and is likely to be more accurate than some of the nautical charts
published many years ago and the variations mentioned on their roses are for the
dates of publication of the chart.
Plotting Charts or Sheets
These are blank sheets of charts on which only the latitudes are marked. The
longitude scale is not marked and can, thus, be used for plotting in any longitude
for the particular marked latitude. This is so because on a Mercator chart the
8
 longitude scale is constant while the latitude scale varies as per the meridonial Charts, Datum and
parts. These charts are for 0 to 4 degrees, 4 to 11 degrees and so on. These are used Compass Corrections
primarily for plotting position lines to obtain the ships position using celestial
navigation.

10.6 INDIAN AND ADMIRALTY CHART

CATALOGUE AND FOLIO SYSTEM
“ Part 1 of Regulation 9 of SOLAS Chapter V states –
Contracting Governments undertake to arrange for the collection and compilation of
hydrographic data and the publication, dissemination and keeping update of all nautical
information necessary for safe navigation.
Every nation’s Hydrographic office surveys waters/seas/oceans around its coastline and
makes/publishes/updates/maintains charts and publications in its folio system like
Admiralty (UK), Hydrographer (USA), National Hydrographic office, Dehradun, India.
Arrangements of charts on board is done as per folio systems of Admiralty or Indian or
company could have its own chart folio system, which is done as per geographical areas.
Folio nos. can be found in respective chart catalogues. Charts may also be arranged in
numerical order on certain ships. You must know the folio system on board your ship.
The list of charts and publications on board with their update status is maintained in chart
folio index.
Indian Chart Catalogue
The abridged Indian chart catalogue is available from national hydrographic office
Dehradun and also with chart agents.
Subsequent corrections may be made from Indian Notices to Mariners. The
Notices to Mariners also contains information regarding all the chart and
Hydrographic publications.
Purchase of Charts and Publications may be made from the National Hydrographic
Office, Dehradun and Chart Agents as listed in Chart Catalogue and Naval Chart
Depot, Mumbai.
Arrangements of navigational charts is in numerical sequence.
Description of Chart
Chart Number is printed in one of the corners of the limits of a chart in the index.
Where the limits are not shown, the chart number is given against the name of the
place.
The Titles are those given on the Chart. The title of the coastal chart or general title
is given first, followed by the titles of any plans, which are usually of places within
the area of the coastal chart. Alternative or secondary names are shown in brackets.
As far as possible, titles of plans are shown in the order, in which they appear in
Sailing Directions. The Scale of a Chart is shown against its title in the text.
The date of New Edition of a Chart, if published, is shown in the text, in addition
to the original date of publication.
Publications of Indian Hydrographic Office
Indian Notices to Mariners –
Indian Hydrographer issues Notices to Mariners like Admiralty but they are issued
every fortnight and not weekly. Indian Notices to Mariners are available for
consultation at most of the Indian Ports and some of the foreign ports.
Following publications are available and are being published by National
Hydrographic Office Dehradun on a regular basis.
9
Practical Navigation Sl. No. INP No. Description
1. INP 1 West Coast of India Pilot
2. INP 1s Supplement to West Coast of India Pilot
3. INP 10 Catalogue of Indian Charts and Publications
4. INP 11 Nautical Almanac
INP 21 Indian List of Lights Volume F and K
5. INP 21(9) Indian List of Lights Volume D and E
6. INP 61 IALA Maritime Buoyage System
7. INP 400 Sight Forms (Book of 24 Forms)
8. Chart 5020 Symbols and Abbreviations

Admiralty Chart Catalogue and Folio System

Admiralty charts are published by United Kingdom Hydrographic Office (UKHO).
The UKHO produces an unrivalled worldwide series of over 3,300 paper charts,
offering global coverage at a range of scales. They can be kept on-board as per
Admiralty chart folios.
Catalogue of Admiralty Charts and Publications (NP131) gives information on:
• A comprehensive reference in graphical and textual form of all admiralty
charts and publications worldwide.
• Listed by region for easy reference with a composite index.
• Full details of individual title, scale, coverage and UK recommended retail
price for each chart and publication.
• Includes details of Admiralty distributors worldwide.
• Includes details of electronic charts – ARCS.
• Essential work of reference for Admiralty Chart users.
• Updated and published annually.

10.7 CORRECTIONS OF CHARTS AND OTHER

PUBLICATIONS
Charts and publications including List of Lights, List of Radio Signals, Sailing Directions
and other Admiralty publications must be kept updated/corrected at all times. Charts and
publications relevant for the voyages intended must be ready updated/corrected prior the
commencement of the voyage.
A standard outfit of charts will be kept in a Chart management system that includes
arrangements for the supply of charts, new editions of charts and extra charts, as
required.
Admiralty Notices to Mariners notifies whenever a new edition of chart or any book or
publication is published, same must be obtained as soon as possible. Subsequent
corrections refer to the new edition.
Regular supply of Radio Navigational Warning and Notices to Mariners, must be
arranged.
A system of documentation is required that shows quickly and clearly that all relevant
notices have been received and corrections done, and that new charts, new edition of
charts, latest edition of the publications and their supplements have been obtained or
ordered.
10
Chart Folio index contains sheets providing a numerical index of charts, indicates in Charts, Datum and
which folio they are held, and has space against each chart for logging Notices to Compass Corrections
Mariners affecting it.
It is divided into three parts :
Part – I Navigational Charts (including Decca, Omega and Loran – C)
Part – II Admiralty reproductions of Australian and New Zealand charts
Part – III Miscellaneous charts
10.7.1 Chart Corrections
Consult the index of charts affected in the weekly edition of Notices to Mariners
containing the last Notice to which the outfit has been corrected, and all subsequent
weekly editions. If any charts held are mentioned, enter the numbers of the notices
affecting them against the charts concerned in the log, and then correct the charts.
Enter the number of the notice to which the outfit has been corrected in the Chart
Correction Log as maintained in the Chart Folio Index. Insert the Folio number on the
thumb label of each chart. If not using standard Admiralty Folios, enter the Folio Number
against each chart of the Log.
No correction, except those given in Section II of Admiralty Notices to Mariners, should
be made to any chart in ink.
All corrections given in Notices to Mariners should be inserted on the charts affected.
When they have been completed the numbers of the notices should be entered clearly and
neatly; permanent notices in waterproof violet ink, T and P (Temporary and preliminary
notices) in pencil.
Consult the latest monthly notice listing Temporary and preliminary notices in each
weekly edition subsequent to it. If any charts are affected by those notices, enter in pencil
the numbers of the notices against the charts in the log, and then correct the charts for
them (also in pencil).
Extract the T and P notices from weekly editions subsequent to the current Annual
Summary of Admiralty Notices to Mariners and make them into a T and P notices file.
Corrections to charts from information received from authorities other than the
Hydrographic Dept. may be noted in pencil, but no charted danger should be expunged
without the authority of the Hydrographer of the Navy.
T and P notices should be rubbed out as soon as the notices are received cancelling them.
Symbols and Abbreviations used on Admiralty Charts should be followed to ensure
uniformity of corrections. These symbols are invariably indicated on Overlay Correction
Tracings.
If several charts are affected by one notice, the largest scale chart should be corrected
first to appreciate the detail of the correction.
When correcting a chart, first check that the last published correction to it (which is given
at the end of the new notice) has been made to the chart. If the last correction has not
been done on the chart, it must be done first.
Small corrections are only carried out on board the ship and are shown at the
bottom left corner of the chart.
Whenever a correction has been made to a chart the number of the notice and the year
(if not already shown) should be entered in the bottom left-hand corner of the chart, the
entries for permanent notices as small corrections and those for T and P notices in pencil
below the line of small corrections.

10.8 ROTATION OF THE EARTH

Earth is a planet which revolves around the sun. It rotates around its own axis also. The
axis of the Earth is the diameter about which it rotates. The geographic poles of the Earth 11
Practical Navigation are the two points where the axis meets the Earth’s surface. The Earth rotates about its
axis once each day and the direction towards which the Earth rotates is called East. West
is the direction 180° away from East, North is the direction 90° to the left of East, and
South is the direction 90° to the right of East. The two poles of the Earth are designated
as North Pole and South Pole, accordingly.

10.9 DIRECTION ON THE SURFACE OF THE

EARTH
Compasses provide direction and correct direction is the core of navigation. A reference
system of some kind is needed for obtaining direction. On shore we have various road
signs for reference. At sea, where landmarks are not available, we need some other
reference for obtaining direction.
The early man used the sun at sunrise, and at sunset as reference for direction; he also
used the stars to identify the north (the Pole star) and south (the Southern Cross) and the
other constellations at different times to figure out the directions. He later found that a
magnet always points toward a fixed direction, thus was born the idea of a compass based
on the magnetic property of the Earth. And then Sperry invented Gyro-compass.

10.9.1 Direction Indicated by Gyro Compass

In an effort to develop a navigational instrument whose accuracy would be unaffected by
stray magnetic fields, the gyrocompass, which does not use magnetism, was developed.
Gyrocompass (Figure 10.2) is used in modern navigation systems because it can be set in
a direction which is very close to true North rather than the Magnetic north. All ships
more than 150 GT are required to carry both magnetic compass and gyrocompass.
The gyrocompass always points to a fixed direction which is normally referred to gyro
North.

Figure 10.2 : Gyrocompass

10.9.2 Direction Indicated by Magnetic Compass

A magnetic compass is reliable and indicates directions with respect to Magnetic North.
A magnetic compass works at all times and in most places.
Difference between Compass and the true North is compass error and can be found by
finding variation at a place and deviation from the deviation table.
The needle of the Standard compass on a ship points to a direction called compass north
which could be few degrees to the left or to the right of true north.
This angle between true north and compass north depends on the Earth’s magnetic field
at that place and ships’s own magnetic field.
12
 Charts, Datum and
Compass Corrections

Figure 10.3: Ship’s Magnetic Compass

10.9.3 Direction of Ship’s Head on Gyro Compass (Gyro Course)

The gyro course is the angle between the gyro north and ship’s fore and aft line (ship’s
head) measured clockwise from gyro north and expressed in 360° notation (Figure 10.4).
The ship’s heading marker in the gyro as well as magnetic compass is shown as a mark
or line called lubber line.

GN
GN

Gyro Course
o
205

Gyro Course
o
120

Figure 10.4

10.9.4 Magnetic Compass (Compass Course)

The compass course is the angle between compass north and ship’s fore and aft line
(ship’s head) measured clockwise from compass north and expressed in 360o notation.
True course is the angle between true north and ship’s fore and aft line (ship’s head)
measured clockwise from true north and expressed in 360o notation.
True and compass courses can also be expressed with reference to the meridian
(N-S line) in quadrantal notation. For example, a true course of 120o(T) can also be
expressed as S60oE, and true course of 205o(T) can be expressed as S25oW.

10.10 MEASURING DISTANCE BETWEEN TWO

POINTS ON MERCATOR CHART
Unit of distance used at sea is nautical mile. One nautical mile is equal to one minute of
the arc on the latitude scale. Distance on Mercator chart is measured using the dividers on
the scale of the latitude between two positions.
The distance between any two positions must be measured in the region of the mean
latitude between these positions. When large distances are involved, set the dividers to a
convenient distance, e.g. 10 miles, or 20 miles along the mean Latitude; step off along
the course line making a note of the number of steps taken, measuring the final position
of a step separately. 13
Practical Navigation
10.11 TRUE, MAGNETIC AND COMPASS NORTH
VARIATION AND DEVIATION
10.11.1 True Meridian and True North
A true meridian is a semi-great circle, passing through the geographical north and south
poles of the earth and cutting the Equator at right angles.
True north is a line on surface of earth joining south geographical pole to north
geographical pole. True north and true meridian, therefore, lie on same line.
10.11.2 Magnetic Meridian and Magnetic North
Magnetic meridian is a semi-great circle passing through the magnetic north and
magnetic south poles of the earth.
It is the direction in which a magnetic needle lies when freely suspended, and acting
under influence of the earth’s magnetism only.
Magnetic north is a line joining south magnetic pole of the earth to the north magnetic
pole. So the magnetic meridian and magnetic north lie on the same line and in the same
direction.
10.11.3 Compass North
The needle of the standard (magnetic) compass on a ship does not point towards either
magnetic north or true north. Instead it points towards a direction called compass north
which could be a few degrees to the left or right of magnetic or true north. This is due to
the effect of ship’s magnetic field on the compass.
10.11.4 Variation
Variation is the angle between the true and the magnetic north that is to say, the angle
which the freely suspended magnetic needle on surface of the earth makes with the true
meridian.

TN MN
MN

Var-W Var-E

Figure 10.5
If the magnetic needle is drawn to the right of the true meridian, the variation is said to be
EASTERLY and if the magnetic needle is drawn to the left of the true meridian, the
variation is termed WESTERLY.
Variation differs from place to place but is constant for different ship’s head, i.e. it does
not vary with the course of the ship.
10.11.5 Deviation
Due to the earth’s magnetism, the vessel (which is built mainly of steel) also acquires a
certain amount of magnetism and, thus, the compass needle of the ship’s standard
magnetic compass when placed on the ship does not lie in the magnetic meridian, but
may be deflected to one side or the other from it. The angle which the compass needle
14 makes with the magnetic north is known as the “Deviation”.
 If the compass needle is drawn to the right of the magnetic meridian, the deviation is Charts, Datum and
termed Easterly and if the compass needle is drawn to the left of the magnetic meridian, Compass Corrections
the deviation is termed Westerly.
The deviation varies in name and amount, as the ship’s head turns in azimuth as
temporary magnetism attained by the vessel varies with ship’s head. Magnetism acquired
by the ship temporarily changes with the change in course.
The value of the deviation, on the different courses or ship’s head, usually 10o apart, is
ascertained by the observations and a table called “Deviation Card” is drawn up. This
Deviation Card is usually kept pasted at a conspicuous place in the Chart Room on board
the ship.
MN
CN Dev-W Dev-E CN

Figure 10.6

10.12 DEVIATION CARD/TABLE AND VARIATION

FROM CHART
Deviation of the ship’s magnetic compass depends on the ship’s heading (course) by
compass. On every ship, one will find either a deviation card or deviation curve in chart
room to find deviation. For reference, three tables are given for students to solve
examples in the foregoing sections.

Ship’s Head Deviation Deviation Deviation

by Compass Card-I Card-II Card-III
000o 2.0o W 12.5o E 4.6o E
o o o
010 3.5 W 11.5 E 4.4o E
020o 5.5o W 10.5o E 4.1o E
030o 7.0o W 9.0o E 3.7o E
040o 9.0o W 7.0o E 3.1o E
050o 10.0o W 5.0o E 2.8o E
060o 11.5o W 3.0o E 1.6o E
070o 12o W 1.0o E 0.6o E
o o o
080 12.5 W 0.5 W 0.6o W
090o 12.5o W 2.0o W 1.6o W
100o 11.5o W 3.5o W 2.3o W
110o 10.5o W 5.0o W 2.9o W
120o 9.0o W 7.0o W 3.4o W
130o 7o W 8.5o W 3.9o W
140o 5o W 10.0o W 4.1o W 15
Practical Navigation 150o 3o W 11.0o W 4.4o W
160o 1o W 12.0o W 4.6o W
170o 0.5o E 13.0o W 4.6o W
180o 2.0o E 12.5o W 4.5o W
190o 3.5o E 11.5o W 4.4o W
200o 5.00 E 10.00 W 4.30 W
o o o
210 7.0 E 8.0 W 4.0o W
220o 8.5o E 6.5o W 3.7o W
230o 10.0o E 4.5o W 3.4o W
240o 11.0o E 2.5o W 2.0o W
250o 12.0o E 1.0o W 2.5o W
260o 13.0o E 0.5o E 1.9o W
270o 12.5o E 2.0o E 1.2o W
280o 11.5o E 3.5o E 0.4o W
ov o o
290 10 E 5.5 E 0.5o E
300o 8o E 7.0o E 1.3o E
310o 6.5o E 9.0o E 2.1o E
320o 4.5o E 10.0o E 2.8o E
330o 2.5o E 11.5o E 3.4o E
340o 1.0o E 12.0o E 3.9o E
350o 0.5o W 12.5o E 4.3o E
o o o
360 2.0 W 12.5 E 4.6o E

For example, for ship’s course of 035(C), the deviation of compass by using Deviation
card I is 8oW.
Variation is found from chart. It is mentioned on the chart for a particular year along with
annual change and can be found near the compass rose.

10.13 FINDING TRUE COURSE FROM COMPASS

COURSE
Compass error is applied to compass course to get true course. The resultant of the
variation and the deviation is called the compass error. Compass error, thus, is the angle
between the compass needle on board the ship and the true meridian and is computed by
finding the algebraic sum of the Variation and Deviation.
If both Variation and the Deviation are of the same name, add the two and give the same
name to the compass error; and if different in names, subtract the two and give the name
of the higher quantity. This is further explained below :
(a) If Variation is 8oW and Deviation for the ship’s head is 4oW, then the
compass error will be 12oW.
(b) If Variation was 6o E and Deviation for the ship’s head was 3oW, then the
compass error will be 3o E.
(c) If Variation is 7oW and Deviation is 12o E, then the compass error will be
5oE.
CN
MN
TN DEV

16 VAR
C. E.
 Charts, Datum and
Compass Corrections

Figure 10.7

MN TN
CN

Magnetic
Course
Var
Dev
True
Course

Compass
Course

Ship’s Head

Figure 10.8
Thumb rule of CADET can be applied in converting Compass courses/bearings to True
courses/bearings and vice-versa. One may also remember “Error East compass Least”
“Error West compass Best”.
For example, if ship’s course is 235(C) and C.E. is 5oE. To get true course, one shall add
Easterly compass Error as per above thumb rule. So the true course is 240(T).

Compass to True

CADET

Add East (C.E.)

Figure 10.9
Note
The compass error is always for a particular ship’s head and not for the bearing.
Example 10.1
Given the ship’s head by Compass 045o (C) find the true course if Variation is
5oW (Deviation Card 1).
Solution
Compass Course 045 o(C)
o o
Deviation 9.50 W (Interpolate between 040 and 050 )
Variation 5o W
Compass Error (C.E.) 14.5 oW
True course 30.5 o(T)
17
Practical Navigation
10.14 FINDING COMPASS COURSE FROM TRUE
COURSE
To find compass course from true course, Thumb rule of cadet can be used as described
earlier.
Example 10.2
If true course is 245°(T), variation is 3°W and deviation is 6°E, find Compass
course.
Solution
Var 3oW
Dev 6o E
C.E. 3oE
True course 245o(T)
We are converting true course to compass course. From above thumb rule, we are going
opposite to arrow so we shall subtract Easterly compass error to get compass course.
∴ Compass course: 242o(C)
If we only knew true course and variation; deviation was not known. Then to find
Compass course we will convert true course to magnetic course after applying variation.
From magnetic course we will use deviation table to get deviation. This deviation can be
applied to magnetic course to get compass course.
Example 10.3
Convert true course of 245o(T) to compass course, variation 3oW and Deviation
card I.
Solution
True course 245o(T)
Variation 3oW
Magnetic course 248o(M) reverse direction of thumb rule.
From Deviation Table I, we get following :
Compass co Dev Magnetic co
o o
230 10 E 240o
240o 11oE 251o
From small table above, after interpolation, we get dev of 10.7oE against magnetic course
of 248°(M). So deviation = 10.7°E
After appling this dev to magnetic course we get 237.3°(C) compass course.

10.15 COMPASS ERROR FROM TRANSIT BEARING

Gyro or standard compass error can also be found on board by taking transit bearing. By
transit it means that the observer and two objects are in same line.
On board ship, one can take bearing of a lighthouse when it is in transit with other
lighthouse or landmark.

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In Figure 10.10, if a ship is steering 030o(C) and at a certain time when she is at point A, Charts, Datum and
she could take bearing of the lighthouses when they are in transit. So let’s say she takes Compass Corrections
transit bearing at A and it is 070°(C).
From the chart in use, one could draw a straight line joining these two Lighthouses and
with the help of parallel ruler will be able to find true bearing from Compass rose.
If true bearing obtained between the two lighthouses from chart is 075°(T), one can now
compare this true bearing with the compass bearing taken.
True bearing when two lighthouses in transit: 075°(T) Compass bearing when these
lighthouses in transit: 070°(C) so compass error: 5°E
Transit
Bearing
Ship’s
Course
Lighthouse
or a Beacon

Lighthouse
or a Beacon

Figure 10.10

10.16 APPLYING COMPASS ERROR TO COMPASS

COURSE/COMPASS BEARING TO FIND TRUE
COURSE/TRUE BEARING
As stated earlier, compass course can be converted to true course by applying compass
error as per cadet thumb rule. Same way compass error can be applied to compass
bearings to get true bearings.
Compass error depends on the ship’s head not on compass bearings. On a certain ship’s
head compass error remains constant but bearings may change.
Conversion of compass course and bearings to true course and bearings is explained by
the following example.
Example 10.4
While steering 120°(C), bearing of a lighthouse “X” was 030°(C) and bearing of
another lighthouse “Y” was 091°(C).
Deviation card I and variation 4°W. Find true course steered and true bearings of
the lighthouses.
Solution
Compass course : 120°(C)
Deviation (Table I) : 9°W
Variation : 4°W
Compass Error : 13°W
True course : 107°(T) (Compass to true add East, Subtract West)
Likewise by applying the same rule true bearings of the lighthouses X and Y will
be 017o(T) and 078 o(T) respectively.
SAQ 1
(a) Convert 276(C) compass course to true course, variation 4oW, Deviation III.
(b) Convert 316(T) true course to compass course, variation 4oE, Deviation III.
(c) A vessel on a course of 055o(C) compass course observes lighthouse “A” is
bearing 120(C) and light “B” is bearing 171o(C). Dev card II,
variation = 3°W. Find true course and true bearings of two lighthouses. 19
Practical Navigation (d) A Vessel was heading 190o(G) and 199o(C). At 1300 hrs bearing of a
lighthouse was 031(G) when in transit with a beacon. True bearing of this
lighthouse when in line with beacon from Chart was 032°(T). Find gyro
compass error and standard compass error. Find also the deviation of
compass if variation was 2oW.

10.17 SUMMARY
Chart is a projection of earth’s surface onto flat surface. For navigation, normally
Mercator charts are used.
Nautical charts can be classified into three categories
(a) Ocean Charts
(b) Coastal Charts
(c) Plan Charts
Charts are published by UK HMSO (Admiralty charts) and Indian hydro-graphic office.
Weekly notices to mariners are published by Admiralty and Indian hydro-graphic
department. All Charts on board must be kept in a chart folio system and shall be kept
corrected to latest notice to mariners.
All these small corrections numbers are written on bottom left corner of the chart and a
chart correction log is maintained on board.
On board ship, the direction in which the ship is sailing is determined by compass. On
ships, there are normally two types of compasses – standard magnetic compass and
gyrocompass. Needle of the standard magnetic compass always points to compass North
and the angle between compass North and ship’s head measured clockwise from compass
North is called Compass course. To plot on the chart one needs to find true course. To
convert compass course to true course one must know compass error.
Compass error is the angle between True North and compass North. It is also algebraic
sum of deviation and variation.
Deviation is the angle between Magnetic North and compass North. It is named E or W
as explained earlier.
Variation is the angle between True North and Magnetic North. It is named E or W as
explained earlier.
Gyro compass settles to point to a direction called gyro North. Angle between Gyro north
and ship’s head measured clockwise from gyro North is called Gyro course. Gyro error is
applied to gyro course to get True course. Gyro error is the angle between True North
and Gyro North and is named low (L) or high (H).
Both compass error and gyro compass error can also be found by taking transit bearing or
celestial observation on board-ship.

10.18 KEY WORDS

Great Circle : It is a circle on surface of sphere whose plane passes
through its centre.
Small Circle : It is a circle on surface of sphere whose plane does
not pass through its centre.
20
Chart : Chart is a projection of earth’s surface of coastal and Charts, Datum and
Compass Corrections
ocean areas on a flat paper.
The Natural Scale of a Chart : It is the ratio that the distance between two points on
the chart bears to the actual distance between them on
the earth.
Magnetic North : It is the direction in which a magnetic needle lies
when freely suspended, and acting under influence of
the earth’s magnetism only.
Compass North : The needle of the standard compass on a ship points
to a direction called compass north which could be
few degrees to the left or to the right of true north.
True North : True north is a line on surface of earth joining south
geographical pole to north geographical pole.
Essentially true north and true meridian lie on the
same line.
Variation : It is the angle between True North and magnetic
North.
Deviation : It is the angle between magnetic North and compass
North.
Compass Error : It is the angle between true North and compass North
and also can be defined as algebraic sum of variation
and deviation.
Compass Course : The compass course is the angle between compass
North and ship’s fore and aft line (ship’s head)
measured clockwise from compass North and
expressed in 360o notation.
True Course : True course is the angle between true North and
ship’s fore and aft line (ship’s head) measured
clockwise from true north and expressed in 360o
notation.
Gyro Compass Course : Gyro course is the angle between Gyro North and
ship’s fore and aft line (ship’s head) measured
clockwise from gyro North and expressed in 360o
notation.
Bearing : Bearing is the angle at the observer between the North
(it could be True north, magnetic north or compass
north) and line joining the terrestrial landmark or
celestial body and observer, measured clockwise from
reference North expressed in 360o notation or
quadrantal system.
True Bearing : If reference north is Geographic north (also called
true north), bearing is said to be true bearing.
Magnetic Bearing : If reference north is magnetic north, bearing is said to
be magnetic bearing.
Compass Bearing : If reference north is compass north, bearing is said to
be compass bearing.

10.19 ANSWERS TO SAQs

SAQ 1
(a) True course 271 o (T)
(b) Compass Course 309.9 o(C) 21
Practical Navigation (c) True course 056 o (T), true bearing of “A” - 121°(T) and of “B” - 172°(T)
(d) Gyro error 1°L, Compass error 7°W, Deviation 5°W.

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