Position and Direction on the Earth

    Navigation is a process of planning and carrying out the movement of transport of all kinds from one place to another at sea, in the air, on land or in space. Navigation of all waterborne crafts called Marine Navigation.

Shape of Earth

    The Earth is flattened at poles and shape of the Earth is called a Oblate spheroid. Radius of Earth at equator about 3444 n mile (Nautical Mile) and at poles about 3432 n mile. The Earth revolves about axis from West to East as a result heavenly bodies are appeared rise in East and set in West.

    The Great Circle is an imaginary circle on surface of sphere its plain pass through the center of sphere  (shortest distance between two points of the sphere)

    The Small Circle is an imaginary circle on earth surface its plain doesn’t pass through center of sphere (All latitudes except equator)

    Meridians are semi great circles joining two poles on the Earth. Prime Meridian (Greenwich Meridian) is meridian which passes through Greenwich. Rhumb line is a line which cuts all meridians at same angle on earth surface.

    Latitude is known as the angle between the equator plane and the line perpendicular to the Earth’s surface at observers position.

All latitudes except 0⁰ are small circles.

    Longitude is known as the angle between plane of Greenwich meridian and meridian which goes through observers position measured 0⁰ to 180⁰ East or West of Greenwich.

Sea mile is the length of 1’ of arc measured along the meridian.

'M' the position

'C' center of curvature at 'M'

AMB is the arc of 1’ at 'C'

    Earth is flattened at poles so sea mile isn’t constant. At equator it is 1842.9 m, at poles it is 1861.7 m and at 45⁰ it is 1852.3 m. International nautical mile is a standard fixed length (1852 m) used in publications.

    Speed of ships are measured in Knots. Knot (kn) means one international nautical mile per hour. (1 kn = 1.15 miles per hour)

1n mile = 10c (cable) = 200 yard

    Linear Latitude is the Length of arc of meridian between equator and observers place, measured in sea miles N or S of Equator.

    eg – 80 30’ N, 760 15’ E is (8 X 60)+30=478 sea mile North of Equator

GX

= 730

= 73 X 60

=4380 Geographical mile measured along equator to East

GX appears shorter.

    True North is known as the Northerly direction of meridians, which is used to measure true bearing and course. True bearing is the angle between the meridian and direction of the object, measured clockwise from 000 to 360 degrees.

    Apart from that true course is the direction on earth’s surface which ship is steered.

Types of North

• True North -  Northerly direction of meridian, which true bearing and course measured
• Gyro North
• Magnetic North
• Magnetic Compass North

Gyro Error

• Error High - Subtracted from gyro bearing.
• Error Low - Added to gyro bearing.

Magnetic North

    Magnetic lines of force emanate from South pole and follow a semi great circle path to North. These lines are irregular because Earth’s magnetic field is irregular. Magnetic North means direction which north ends of magnetic needle remains horizontal under Earth’s magnetism only.

Variation

    Angle between geographical (true) north and magnetic north which measured East or West from true north. It changes from place to place and same place with time as Earth’s magnetic field is irregular. Ships are made of Iron, machinery which produces her own magnetic field therefore magnetic north deviates from magnetic meridian (Compass North)

Deviation

    Angle between magnetic meridian and compass North measured East or West from magnetic North.

TRUE + / - VARIATION = MAGNETIC

MAGNETIC + / - DEVIATION = COMPASS

Converting magnetic / compass course to true course. Use CADET rule mentioned below.

C             AD    E       T

compass  add    east   true

Apply the error in following sequence.

C             D              M             V             T

compass   deviation   magnetic   variation   true

can          dead         man          vote         twice