Chapt-01.pdf

CHAPTER 1

INTRODUCTION TO MARINE NAVIGATION

DEFINITIONS

100. The Art And Science Of Navigation

with frequent or constant determination of position

relative to nearby geographic and hydrographic

features.

Marine navigation blends both science and art. A good

navigator constantly thinks strategically, operationally, and

tactically. He plans each voyage carefully. As it proceeds,

he gathers navigational information from a variety of

sources, evaluates this information, and determines his

ship’s position. He then compares that position with his

voyage plan, his operational commitments, and his pre-

determined “dead reckoning” position. A good navigator

anticipates dangerous situations well before they arise, and

always stays “ahead of the vessel.” He is ready for naviga-

tional emergencies at any time. He is increasingly a

manager of a variety of resources--electronic, mechanical,

and human. Navigation methods and techniques vary with

the type of vessel, the conditions, and the navigator’s

experience. The navigator uses the methods and techniques

best suited to the vessel, its equipment, and conditions at

hand.

Some important elements of successful navigation

cannot be acquired from any book or instructor. The science

of navigation can be taught, but the art of navigation must

be developed from experience.

• Celestial navigation involves reducing celestial

measurements taken with a sextant to lines of

position using calculators or computer programs, or

by hand with almanacs and tables or using spherical

trigonometry.

• Radio navigation uses radio waves to determine

position through a variety of electronic devices.

• Radar navigation uses radar to determine the

distance from or bearing of objects whose position is

known. This process is separate from radar’s use in

collision avoidance.

• Satellite navigation uses radio signals from

satellites for determining position.

Electronic systems and integrated bridge concepts are

driving navigation system planning. Integrated systems

take inputs from various ship sensors, electronically and

automatically chart the position, and provide control

signals required to maintain a vessel on a preset course. The

navigator becomes a system manager, choosing system

presets, interpreting system output, and monitoring vessel

response.

In practice, a navigator synthesizes different method-

ologies into a single integrated system. He should never

feel comfortable utilizing only one method when others are

also available. Each method has advantages and

disadvantages. The navigator must choose methods

appropriate to each situation, and never rely completely on

only one system.

With the advent of automated position fixing and

electronic charts, modern navigation is almost completely

an electronic process. The mariner is constantly tempted to

rely solely on electronic systems. But electronic navigation

systems are always subject to failure, and the professional

mariner must never forget that the safety of his ship and

crew may depend on skills that differ little from those

practiced generations ago. Proficiency in conventional

piloting and celestial navigation remains essential.

101. Types of Navigation

Methods of navigation have changed throughout

history. New methods often enhance the mariner’s ability to

complete his voyage safely and expeditiously, and make his

job easier. One of the most important judgments the

navigator must make involves choosing the best methods to

use. Each method or type has advantages and

disadvantages, while none is effective in all situations.

Commonly recognized types of navigation are listed below.

• Dead reckoning (DR) determines position by

advancing a known position for courses and

distances. A position so determined is called a dead

reckoning (DR) position. It is generally accepted that

only course and speed determine the DR position.

Correcting the DR position for leeway, current

effects, and steering error result in an estimated

position (EP) .

• Piloting involves navigating in restricted waters

INTRODUCTION TO MARINE NAVIGATION

102. Phases of Navigation

The navigator’s position accuracy requirements, his fix

interval, and his systems requirements differ in each phase.

The following table can be used as a general guide for

selecting the proper system(s).

Four distinct phases define the navigation process. The

mariner should choose the system mix that meets the

accuracy requirements of each phase.

• Inland Waterway Phase : Piloting in narrow canals,

channels, rivers, and estuaries.

Inland

Harbor/

Approach

Coastal

Ocean

• Harbor/Harbor Approach Phase : Navigating to a

harbor entrance through bays and sounds, and

negotiating harbor approach channels.

Piloting

Celestial

Radio

• Coastal Phase : Navigating within 50 miles of the

coast or inshore of the 200 meter depth contour.

Radar

Satellite

• Ocean Phase : Navigating outside the coastal area in

the open sea.

Table 102. The relationship of the types and phases of

navigation. * With SA off and/or using DGPS

NAVIGATION TERMS AND CONVENTIONS

103. Important Conventions and Concepts

observatory as a reference. The publication by the

Observatory of the first British Nautical Almanac in 1767

further entrenched Greenwich as the prime meridian. An

unsuccessful attempt was made in 1810 to establish

Washington, D.C. as the prime meridian for American

navigators and cartographers. In 1884, the meridian of

Greenwich was officially established as the prime meridian.

Today, all maritime nations have designated the Greenwich

meridian the prime meridian, except in a few cases where

local references are used for certain harbor charts.

Charts are graphic representations of areas of the

Earth, in digital or graphic form, for use in marine or air

navigation. Nautical charts, whether in digital or paper

form, depict features of particular interest to the marine

navigator. Charts have probably existed since at least 600

B.C. Stereographic and orthographic projections date from

the 2nd century B.C. In 1569 Gerardus Mercator published

a chart using the mathematical principle which now bears

his name. Some 30 years later, Edward Wright published

corrected mathematical tables for this projection, enabling

other cartographers to produce charts on the Mercator

projection. This projection is still the most widely used.

Sailing Directions or pilots have existed since at least

the 6th century B.C. Continuous accumulation of naviga-

tional data, along with increased exploration and trade, led

to increased production of volumes through the Middle

Ages. “Routiers” were produced in France about 1500; the

English referred to them as “rutters.” In 1584 Lucas

Waghenaer published the Spieghel der Zeevaerdt (The

Mariner’s Mirror) , which became the model for such

publications for several generations of navigators. They

were known as “Waggoners” by most sailors.

The compass was developed about 1000 years ago.

The origin of the magnetic compass is uncertain, but

Throughout the history of navigation, numerous terms

and conventions have been established which enjoy

worldwide recognition. The professional navigator, to gain

a full understanding of his field, should understand the

origin of certain terms, techniques, and conventions. The

following section discusses some of the important ones.

Defining a prime meridian is a comparatively recent

development. Until the beginning of the 19th century, there

was little uniformity among cartographers as to the

meridian from which to measure longitude. But it mattered

little because there existed no method for determining

longitude accurately.

Ptolemy, in the 2nd century AD, measured longitude

eastward from a reference meridian 2 degrees west of the

Canary Islands. In 1493, Pope Alexander VI established a

line in the Atlantic west of the Azores to divide the

territories of Spain and Portugal. For many years, cartog-

raphers of these two countries used this dividing line as the

prime meridian. In 1570 the Dutch cartographer Ortelius

used the easternmost of the Cape Verde Islands. John

Davis, in his 1594 The Seaman’s Secrets , used the Isle of

Fez in the Canaries because there the variation was zero.

Most mariners paid little attention to these conventions and

often reckoned their longitude from several different capes

and ports during a voyage.

The meridian of London was used as early as 1676, and

over the years its popularity grew as England’s maritime

interests increased. The system of measuring longitude both

east and west through 180

may have first appeared in the

middle of the 18th century. Toward the end of that century,

as the Greenwich Observatory increased in prominence,

English cartographers began using the meridian of that

INTRODUCTION TO MARINE NAVIGATION

Norsemen used it in the 11th century, and Chinese

navigators used the magnetic compass at least that early and

probably much earlier. It was not until the 1870s that Lord

Kelvin developed a reliable dry card marine compass. The

fluid-filled compass became standard in 1906.

Variation was not understood until the 18th century,

when Edmond Halley led an expedition to map lines of

variation in the South Atlantic. Deviation was understood

at least as early as the early 1600s, but adequate correction

of compass error was not possible until Matthew Flinders

discovered that a vertical iron bar could reduce certain

types of errors. After 1840, British Astronomer Royal Sir

George Airy and later Lord Kelvin developed

combinations of iron masses and small magnets to

eliminate most magnetic compass error.

The gyrocompass was made necessary by iron and

steel ships. Leon Foucault developed the basic gyroscope in

1852. An American (Elmer Sperry) and a German (Anshutz

Kampfe) both developed electrical gyrocompasses in the

early years of the 20th century. Ring laser gyrocompasses

and digital flux gate compasses are gradually replacing

traditional gyrocompasses, while the magnetic compass

remains an important backup device.

The log is the mariner’s speedometer. Mariners

originally measured speed by observing a chip of wood

passing down the side of the vessel. Later developments

included a wooden board attached to a reel of line. Mariners

measured speed by noting how many knots in the line

unreeled as the ship moved a measured amount of time;

hence the term knot. Mechanical logs using either a small

paddle wheel or a rotating spinner arrived about the middle

of the 17th century. The taffrail log still in limited use today

was developed in 1878. Modern logs use electronic sensors

or spinning devices that induce small electric fields propor-

tional to a vessel’s speed. An engine revolution counter or

shaft log often measures speed aboard large ships. Doppler

speed logs are used on some vessels for very accurate speed

readings. Inertial and satellite systems also provide highly

accurate speed readings.

The Metric Conversion Act of 1975 and the Omnibus

Trade and Competitiveness Act of 1988 established the

metric system of weights and measures in the United

States. As a result, the government is converting charts to

the metric format. Notwithstanding the conversion to the

metric system, the common measure of distance at sea is the

nautical mile .

The current policy of the National Imagery and

Mapping Agency (NIMA) and the National Ocean

Service (NOS) is to convert new compilations of

nautical, special purpose charts, and publications to the

metric system. All digital charts use the metric system.

This conversion began on January 2, 1970. Most modern

maritime nations have also adopted the meter as the

standard measure of depths and heights. However, older

charts still on issue and the charts of some foreign

countries may not conform to this standard.

The fathom as a unit of length or depth is of obscure

origin. Posidonius reported a sounding of more than 1,000

fathoms in the 2nd century B.C. How old the unit was then

is unknown. Many modern charts are still based on the

fathom, as conversion to the metric system continues.

The sailings refer to various methods of mathemat-

ically determining course, distance, and position. They

have a history almost as old as mathematics itself. Thales,

Hipparchus, Napier, Wright, and others contributed the

formulas that permit computation of course and distance by

plane, traverse, parallel, middle latitude, Mercator, and

great circle sailings.

104. The Earth

The Earth is an irregular oblate spheroid (a sphere

flattened at the poles). Measurements of its dimensions and

the amount of its flattening are subjects of geodesy.

However, for most navigational purposes, assuming a

spherical Earth introduces insignificant error. The Earth’s

axis of rotation is the line connecting the north and south

geographic poles.

A great circle is the line of intersection of a sphere and

a plane through its center. This is the largest circle that can

be drawn on a sphere. The shortest line on the surface of a

sphere between two points on the surface is part of a great

circle. On the spheroidal Earth the shortest line is called a

geodesic. A great circle is a near enough approximation to

Figure 104a. The planes of the meridians at the polar axis.

INTRODUCTION TO MARINE NAVIGATION

a geodesic for most problems of navigation. A small circle

is the line of intersection of a sphere and a plane which does

not pass through the center. See F igure 104a.

The term meridian is usually applied to the upper

branch of the half-circle from pole to pole which passes

through a given point. The opposite half is called the lower

branch.

. It is designated east (E) or west (W) to

indicate the direction of measurement.

The difference of longitude (DLo) between two

places is the shorter arc of the parallel or the smaller angle

at the pole between the meridians of the two places. If both

places are on the same side (east or west) of Greenwich,

DLo is the numerical difference of the longitudes of the two

places; if on opposite sides, DLo is the numerical sum

unless this exceeds 180

minus the sum.

The distance between two meridians at any parallel of

latitude, expressed in distance units, usually nautical miles,

is called departure (p, Dep.) . It represents distance made

good east or west as a craft proceeds from one point to

another. Its numerical value between any two meridians

decreases with increased latitude, while DLo is numerically

the same at any latitude. Either DLo or p may be designated

east (E) or west (W) when appropriate.

, when it is 360

106. Distance on the Earth

Figure 104b. The equator is a great circle midway

between the poles.

Distance, as used by the navigator, is the length of the

rhumb line connecting two places. This is a line making

the same angle with all meridians. Meridians and parallels

which also maintain constant true directions may be con-

sidered special cases of the rhumb line. Any other rhumb

line spirals toward the pole, forming a loxodromic curve

or loxodrome . See Figure 106 . Distance along the great

A parallel or parallel of latitude is a circle on the

surface of the Earth parallel to the plane of the equator.

It connects all points of equal latitude. The equator is a

great circle at latitude 0

. See Figure 104b. The poles are

single points at latitude 90

. All other parallels are small

circles.

105. Coordinates

at the poles. It is

designated north (N) or south (S) to indicate the direction of

measurement.

The difference of latitude ( l , DLat.) between two

places is the angular length of arc of any meridian between

their parallels. It is the numerical difference of the latitudes

if the places are on the same side of the equator; it is the sum

of the latitudes if the places are on opposite sides of the

equator. It may be designated north (N) or south (S) when

appropriate. The middle or mid-latitude (Lm) between

two places on the same side of the equator is half the sum

of their latitudes. Mid-latitude is labeled N or S to indicate

whether it is north or south of the equator.

The expression may refer to the mid-latitude of two

places on opposite sides of the equator. In this case, it is

at the equator to 90

Figure 106. A loxodrome.

equal to half the difference between the two latitudes and

takes the name of the place farthest from the equator.

Longitude (l, long.) is the angular distance between

the prime meridian and the meridian of a point on the Earth,

measured eastward or westward from the prime meridian

through 180

Coordinates of latitude and longitude can define any

position on Earth. Latitude (L, lat.) is the angular distance

from the equator, measured northward or southward along

a meridian from 0

INTRODUCTION TO MARINE NAVIGATION

circle connecting two points is customarily designated

great-circle distance . For most purposes, considering the

nautical mile the length of one minute of latitude introduces

no significant error

Speed (S) is rate of motion, or distance per unit of time.

A knot (kn.) , the unit of speed commonly used in

navigation, is a rate of 1 nautical mile per hour. The

expression speed of advance (SOA) is used to indicate the

speed to be made along the intended track. Speed over the

ground (SOG) is the actual speed of the vessel over the

surface of the Earth at any given time. To calculate speed

made good (SMG) between two positions, divide the

distance between the two positions by the time elapsed

between the two positions.

. In this case it is designated

course angle (C) and should be properly labeled to indicate

the origin (prefix) and direction of measurement (suffix).

Thus, C N35

or 180

E=Cn035

(000

+35

), C N155

W=Cn

205

(360

- 155

), C S47

E = Cn 133

(180

-47

). But Cn

260

may be either C N100

W or C S80

W, depending

upon the conditions of the problem.

Track (TR) is the intended horizontal direction of travel

with respect to the Earth. The terms intended track and

trackline are used to indicate the path of intended travel. See

Figure 107a. The track consists of one or a series of course

lines, from the point of departure to the destination, along

which one intends to proceed. A great circle which a vessel

intends to follow is called a great-circle track , though it

consists of a series of straight lines approximating a great circle

Heading (Hdg., SH) is the direction in which a vessel

is pointed at any given moment, expressed as angular

distance from 000

107. Direction on the Earth

Direction is the position of one point relative to

another. Navigators express direction as the angular

difference in degrees from a reference direction, usually

north or the ship’s head. Course (C, Cn) is the horizontal

direction in which a vessel is intended to be steered,

expressed as angular distance from north clockwise through

360

. It is easy to

confuse heading and course. Heading constantly changes as

a vessel yaws back and forth across the course due to sea,

wind, and steering error.

Bearing (B, Brg.) is the direction of one terrestrial

point from another, expressed as angular distance from

000

clockwise through 360

. Strictly used, the term applies to direction through the

water, not the direction intended to be made good over the

ground.The course is often designated as true, magnetic,

compass, or grid according to the reference direction.

Track made good (TMG) is the single resultant

direction from the point of departure to point of arrival at

any given time. Course of advance (COA) is the direction

intended to be made good over the ground, and course over

ground (COG) is the direction between a vessel’s last fix

and an EP. A course line is a line drawn on a chart

extending in the direction of a course. It is sometimes

convenient to express a course as an angle from either north

(North) clockwise through 360

. When measured

from either north or south, it is called

bearing angle (B). Bearing and azimuth are sometimes used

interchangeably, but the latter more accurately refers to the

horizontal direction of a point on the celestial sphere from

a point on the Earth. A relative bearing is measured relative

to the ship’s heading from 000

or 180

(dead ahead) clockwise

. However, it is sometimes conveniently mea-

sured right or left from 000

at the ship’s head through

. This is particularly true when using the table for Dis-

tance of an Object by Two Bearings.

Figure 107a. Course line, track, track made good, and heading.

or south, through 90

through 90

through 360

180

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