NS100

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NS100

• Fundamentals of
• Naval Science

Magnetic Compass
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Objectives
1. Describe the purpose and operation of
the magnetic compass. 2. Discuss magnetic theory
and application. 3. Define, identify, and
solve for variation, deviation, compass
error, magnetic bearings and true bearings.
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Magnetic Compass Theory
Mechanics - the card that shows the
compass bearings has a small magnet which is
attracted to the earths north pole. Physics -
the earths core is made of iron and acts as a
large magnet with it poles in line with the
earths axis. Note the magnetic north pole is
not located at exactly at 90o North
latitude.
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(No Transcript)
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Variation
Magnetic Poles - do not align exactly with the
true poles and their position changes a
little bit from year to year. Magnetic
Meridians - are skewed for true meridians
because of irregularities in the earths
core. True Bearing Magnetic Bearing
Variation ( add East Variation or
subtract West Variation)
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VARIATION
True North Pole
Magnetic North Pole
Notice that the two poles arent together. The
magnetic compass point to the magnetic pole, and
this gives us VARIATION.
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DEVIATION
A normal compass only has one force pulling on
it the magnetic force of the earths core
orienting it towards magnetic north.
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DEVIATION
A ships compass also must deal with magnetic
forces from the ship itself. The sum total of
these forces pulls the compass slightly away from
magnetic north, producing DEVIATION.
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Deviation
Ships Magnetic Field - each steel ship has a
magnetic field that can cause changes in a
magnetic compass. Degaussing - process of
creating a magnetic field to counter the
natural magnetic field of the ships
steel. Compass Card - Gives Deviation based on
whether degaussing is on and heading.
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Compass Error

• Deviation Variation Compass Error
• If observed bearing is Least, Error is East.
• If observed bearing is Best, Error is West.
• Example Deviation is 2o West Variation is
  13o East, what is compass error?
• Answer 13o - 2o 11o East Error

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Applications of Compass Error

• Can Compass Bearing ( o pstgc)
• Dead Deviation
• Men Magnetic Meridian
• Vote Variation
• Twice True Bearing ( o T)
• _at_ Add
• Elections East Error

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Applications of Compass Error Cont.

• Scenario The USS Boxer just lost its gyro
  compass while steering 125o T. The conning
  officer needs to give a magnetic compass course.
  What course do you recommend?
• Given Variation is 8o West from the compass
  rose on the chart, annual increase was added
  to the variation.
• Deviation is 2.5o East from the ships
  compass card for course 133o T.

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Applications of Compass Error Cont.

• Truly True Course
• Valiant Variation
• Marine Mag. Mer.
• Dont Deviation
• Cry Compass
• _at_ Add
• Wedding West

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Applications of Compass Error Cont.

• T 125o T
• V 8o West
• M 125 8 133
• D 2.5o East
• C 133 (- 2.5)
• Ans 130.5o pstgc

Top Conning Officer
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Review Questions
1. Describe how the navigator determines the
variation for his area of operation? 2. State
how to convert from True bearing to compass
bearings? 3. What is magnetic compass error and
how is it determined? 4. Why does a magnetic
compass point north?