1. Welcome to the United States Power Squadrons' Chart Smart Seminar
The Mariner’s Compass
(Section 2)

2. 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

3. 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

4. Basic Compass Idea

6. Magnetic Compass

7. Typical Compass Construction

8. Read a Compass at the Lubber’s Line

9. Compass Installation

10. 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.

11. Variation is the difference between true north and magnetic north
True North Pole
Magnetic North Pole N E S W
Variation is the difference between true north and magnetic north

12. Variation Variation changes with geographic position
True North Pole
Magnetic North Pole N S W E
Variation Variation changes with geographic position

13. 

14. A Chart Compass Rose Variation is found in the center of the

15. A Chart Compass Rose
One or more compass roses are found on each nautical chart

16. 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

17. True vs Steering (Compass) Course
Cross Track Errors
(XTE)
Course
Error (deg.)
XTE 1
(course
distance = 15
mi.) 2
distance = 30 3
distance = 60 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) Course
Variation + Deviation
(deg.)
distance = 15 mi.)
distance = 30 mi.)
distance = 60 mi.)
13 (W or E)
3.5 7 14 60 30
course
error
The purpose of this page is to impress upon the student that if Variation (and Deviation) are not accounted for, then significant cross track errors will occur and the vessel could get into a dangerous situation.
For example - - if there is a combined variation and deviation of 15 degress and a 60 mile track is followed, then the vessel could be as much as miles away from its intended location. See XTE #3 in the table above.

18. 

19. The TVMDC Formula ° True Course T Variation V Magnetic Course M
Deviation D
Compass Course C
Add West
Subtract East
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

20. T _____ V _____ M _____ D _____ C _____ 206
Example #1 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 W
Add West
Subtract East
14 W
220
Compass course to steer is 220 degrees to make good a true course of 206 degrees
220

21. T _____ V _____ M _____ D _____ C _____ 214
Example #2 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 E
14 E
200
A compass course of 200 degrees on the vessel results in a true course of 214 degrees when plotted on the chart
Subtract West
Add East
200

22. End of Section 2
The Mariner’s Compass