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The Mariner’s Compass(Section 2)

Welcome to the

United States Power Squadrons'

Chart Smart Seminar

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Class Agenda

Section 2: Piloting - The Mariner's Compass • How a Compass Works

• Selecting a Compass

• Installing a Compass

• Compass Care

• The Compass Course

• Calculating a Steering Course

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What Does a Mariner's Compass Do ?

• Identifies direction

• Divides direction into degrees

• Shows direction the boat is headed

• Is the link between you and the chart

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Basic Compass Idea

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Magnetic Compass

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Typical Compass Construction

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Read a Compass at the Lubber’s Line

Lubber’s line

Lubber’s line

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Compass Installation

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Compass Care Cover your compass when not in use - sunlight

may damage the card and fluid

Remove your compass during winter storage - freezing may damage the card & bellows resulting in lose of fluid

Remove your compass for trailering

A quality compass, properly taken care of, will last for years

and is an excellent investment.

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Variation

is the difference between true

north and magnetic north

True North Pole

Magnetic North Pole

N E

SW

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True North Pole

Magnetic North Pole

N

S

W

E

Variation

Variation changes with geographic

position

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A Chart Compass Rose

Variation is found in

the center of the

compass rose

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A Chart Compass Rose

One or more compass roses are found on each nautical chart

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Calculating a Steering Course

True is the “Language” of the chart

Magnetic is the “Language” of the compass

The differences between a true course and the steering course must be accounted for to ensure safer boating

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Cross Track Errors (XTE)

Course Error (deg.)

XTE 1 (course distance = 15

mi.)

XTE 2 (course distance = 30

mi.)

XTE 3 (course distance = 60

mi.)

5 1.31 2.62 5.25

10 2.64 5.29 10.58

15 4.02 8.04 16.08

tan. (course error) = XTE / (course distance)

True vs Steering (Compass) CourseVariation + Deviation(deg.)

XTE 1 (course distance = 15 mi.)

XTE 2 (course distance = 30 mi.)

XTE 3 (course distance = 60 mi.)

13 (W or E) 3.5 7 14

60

30

15

XTE 3

XTE 2

XTE 1

course

error

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Deviation is the difference between magnetic and compass course headings due to the boat's magnetic field and is assumed to be 0 for demonstration

The TVMDC Formula

True Course T

Variation V

Magnetic CourseM

Deviation D 0

Compass CourseC

Add West

Subtract East

Subtract West

Add East

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Example #1 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 W

T _____

V _____

M _____

D _____

C _____

206

14 W

220

0

220

Add West

Subtract East

Compass course to steer is 220 degrees to make good a true course of 206 degrees

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Example #2 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 E

T _____

V _____

M _____

D _____

C _____

214

14 E

200

0

200

Subtract West

Add East

A compass course of 200 degrees on the vessel results in a true course of 214 degrees when plotted on the chart

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The Mariner’s Compass

End of Section 2