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Welcome to the
United States Power Squadrons'
Chart Smart Seminar
The Mariner’s Compass
(Section 2)
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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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True North Pole
Magnetic North Pole
Variation
is the difference
between true
north and
magnetic north
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True North Pole
Magnetic North Pole
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 3
60
course
error
30
15
(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
XTE 2
XTE 1
tan. (course error) = XTE / (course distance)
True vs Steering (Compass) Course
Variation + 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
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The TVMDC Formula
True Course
T
Variation
V
Add West
Subtract East
Magnetic Course M
Deviation
D 0
Compass Course C
Subtract West
Add East
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
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T
206
_____
Example #1
notes - deviation is assumed to
be zero for demonstration
only; local variation is
assumed to be 14 W
Compass course to steer is 220
degrees to make good a true
course of 206 degrees
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Add West
V
14 W
_____
M
220
_____
D

0
_____
C
220
_____
Subtract East
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T
214
_____
V
14 E
_____
M
200
_____
D

0
_____
C
200
_____
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Example #2
notes - deviation is assumed to
be zero for demonstration
only; local variation is
assumed to be 14 E
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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Subtract West
Add East
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End of Section 2
The Mariner’s Compass
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