Piloting Compass

1
Piloting Compass Degrees, Chart Plotting,
Relative Bearings

• Ordinary Requirement 9Part I

2
Requirements

• Explain the degree system of compass direction.
  Explain variation and deviation, and show how
  corrections are applied to correcting and
  uncorrecting compass headings.
• Name relative bearings expressed in both degrees
  and points. Be able to report objects in view and
  wind directions with respect to the boat, and
  know the duties of a lookout.
• Name three kinds of devices used aboard ship for
  measuring speed and/or distance traveled and, if
  possible, demonstrate their use.
• Make a dead reckoning table of compass and
  distances (minimum three legs) between two
  points, plot these on a chart, and determine the
  final position.
• References
• Mariners Compass, p. 171

3
Description

• The presentation covers Compass, Plotting a
  Course, Relative Bearings Lookout Duties
• See also
• Piloting 9b Navigation Instruments

4
Mariners Compass

• "Mariner's Compass" is generally taken to mean
  the magnetic compass.
• Historically, it meant the division of the Circle
  of the Horizon into 32 points.
• These points are the directions or rhumbs of the
  wind.
• Thus, a rhumb-line is the direction followed by a
  ship sailing on a straight course.

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Degree System of Direction
90N

• Earth is round not flat
• 360 degrees in a circle
• 60 minutes in a degree
• 60 seconds in a minute
• Divide the Earth
• Parallels (Latitude)
• 0 to 90 North and South
• Meridians (Longitude)
• 0 to 180 East and West

0
(Equator)
90S
180
90E
90W
0
(Prime Meridian)
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Latitude and Longitude

• Latitude
• Parallels
• always written first
• labeled North or South
• 1 degree 60 NM
• 1 minute 1 NM
• Longitude
• Meridians
• Labeled East or West
• Degrees vary in length depending on latitude.

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Examples of Locations

• Kemah Channel Outer Marker
• Lat 2933.1 N
• Lon 9459.5 W
• I Pier, Lakewood Yacht Club
• Lat 2933.6 N
• Lon 9501.9 W

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Compass Variation

• A compass does not normally point to true north.
• The direction in which the compass needle points
  is referred to as magnetic north
• The angle between magnetic north and true north
  is called magnetic variation.
• East of a line from Ohio to Georgia, variation is
  West ()
• West of that line, variation is East (-)
• Usually read from chart (bottom or compass rose)

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Compass Variation

• Variation does not remain constant in time.
• Complex fluid motion in the outer core of the
  Earth causes the magnetic field to change slowly
  with time.
• This change is known as secular variation.

10
Compass Variation

• Variation changes with time in an unpredictable
  manner.
• Variation shown on old topographic, marine and
  aeronautical charts need to be updated if they
  are to be used without large errors.
• Annual change corrections cannot be applied
  reliably if the maps are more than a few years old

11
Magnetic Deviation

• Magnetic deviation is the second correctable
  compass error.
• Deviation is caused by magnetic forces within
  your particular boat
• Deviation varies for each boat.
• Metal objects, such as an engine or an anchor,
  can change magnetic forces.
• Stereo, speakers, and other electric equipment or
  wiring, if too close to the compass, can
  introduce large errors in compass heading.

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Magnetic Deviation

• Deviation changes with heading, resulting in a
  deviation table or deviation chart (shown below).
  
• The vertical axis states the correction in
  degrees.
• The horizontal axis states the ship's heading

13
Deviation Table
14
Magnetic Deviation

• Compensation
• When a compass is newly installed it often shows
  larger deviations then this and needs coarse
  compensation by carefully placing small magnets
  around the compass.
• It is the remaining error that is shown in your
  deviation table.
• Swinging the boat
• Determine deviation by placing your boat in the
  line of a pair of leading lights and turning her
  360 degrees.

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Variation and Deviation

• Three types of 'North'
• True (T)
• Magnetic (M)
• Compass (C)
• Three types of courses
• True Course (TC)
• Magnetic Course (MC)
• Compass Course (CC )
• All are related by deviation and variation.

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Compass Correction

• Converting a true course into a compass course
• T V Makes Dull Children
• True
• Variation
• Magnetic
• Deviation
• Compass
• TC VAR MC DEV CC

• Converting a compass course into a true course
• Can Dead Men Vote Twice
• Compass
• Deviation
• Magnetic
• Variation
• True
• CC - DEV MC - VAR TC

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Compass Correction for Variation

• If we sail a true course of 090 on the chart
  with a variation of 3 East, what is the magnetic
  course?
• Remembering TVM gives
• T 090
• V (-3E)
• M 087
• Remember, by convention
• East is Least (-) West is Best ()

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Correcting for Variation

• Convert a magnetic course of 225 with a
  variation of 3E into a true course
• First, rearrange T V M to give M V T
• M 225
• - V - (-3E)
• T 228
• Remember, by convention
• East is Least (-) West is Best ()

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Correcting for Variation and Deviation

• For a true course of 170, and using the
  deviation table in this presentation, and a
  variation of 15W, what is the compass course?

20
Correcting for Variation and Deviation

• For a true course of 170, and using the
  deviation table in this presentation, and a
  variation of 15W, what is the compass course?
• T 170
• V 15W
• M 185
• D (-3E)
• C 182

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Correcting for Variation and Deviation

• For a compass course of 020, and using the
  deviation table handout and a variation of 15W,
  what is the true course?

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Correcting for Variation and Deviation

• For a compass course of 020, and using the
  deviation table handout and a variation of 15W,
  what is the true course?
• C 020
• - D - 4W
• M 016
• - V - 15W
• T 001

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Compass Rose

• The orientation of a map or chart is shown by a
  compass rose
• May show just north, or it might show many points
  of the compass or degrees.
• May also show a true compass (outer) rose and a
  magnetic compass (inner) rose.

24
Charts and Direction

• To measure direction use a parallel rule.
• Example
• Find the course direction from buoy A to buoy B.

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Charts and Direction

• First align a parallel rule with the two
  positions A and B to establish a line of position
  (LOP).
• Then move one leg of the parallel rule at a time
  walking the rule to the compass rose without
  losing the alignment.
• Finally, when one of the rules is aligned with
  the heart of the compass rose, you can read
  course A-B.
• In this example 153.

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Course Plotting

• Plot of ships progress based on
• Course (assume true unless otherwise noted)
• Speed (in knots)
• Time traveled
• Known starting point (fix)
• Checked and plotted every hour at a minimum
• Checked and plotted at every course or speed
  change

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Terms

• DR Dead Reckoning Position
• EP Estimated Position
• Fix
• LOP Line of Position
• C Course
• S Speed

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To Plot a Course

• Start from a fix
• Label all lines on chart
• Numbers are rounded off
• Use Military time
• Courses bearings are 3 digits (045, 218)
• Label course with C bearing with B (C045)

• Speed is in Knots placed under the course line
• Distance to nearest 1/10 NM place after speed
• DR position is marked by a dot and semicircle
• Fix is marked by a dot and circle with the time
• Estimated position marked by a dot and box

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Labeling Plots
FIX
GPS FIX
DR
C083
1630
1600
1700
S8.4 D3.6
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Plotting Example
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Plotting Practice

• Using the 1998 Bowditch Bay chart and your
  deviation table handout, you depart from the R N
  2 Nun buoy outside Perkins Cove at 0900 on a
  course to R 10 Q R on the left side of Channel
  Island.
• Plot your new course and label the course heading.

• What is your compass course to R 10 Q R ?
• T 287
• V 15W
• M 302
• D (-2E)
• C 300

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Plotting Practice

• You arrive at R 10 Q R at 1124. Depart
  immediately for R N 4 on the Shark River
  channel.
• Plot your new course and label the course heading.

• What is your compass course to R N 4 ?
• T 307
• V 15W
• M 319
• D 0W
• C 319

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Plotting Practice

• You arrive at R N 4 at 1236. Depart
  immediately for RW OR on the Oyster River
  Range, arriving at 1437.
• Plot your new course and label the course
  heading.

• What is your compass course to RW OR ?
• T 145
• V 15W
• M 160
• D (-2E)
• C 158

34
Measuring Distances

• To measure distances on a chart use dividers.
• For short distances, put one point on each end.
• For long distances
• First, take a 'nice' distance like 1' (1 nm) or 5
  ' (5 nm) on the chart scale or use the left and
  right edges (latitude).
• NEVER use the top/bottom edges (longitude).
• Then, start walking with the dividers from one
  spot to the other.
• Finally, adjust the dividers to measure the small
  remainder at its own latitude.

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Calculating Distances

• Now measure and label the distances of the legs
  you plotted on your chart.
• What is the distance from R N 2 outside Perkins
  Cove to R 10 Q R?
• 12.0 nm
• What is the distance from R 10 Q R to R N 4
  on the Shark River channel?
• 7.8 nm
• What is the distance from R N 4 to RW OR on
  the Oyster River range?
• 8.1 nm

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Speed Time Distance
Speed (knots)
Distance (NM)

60 (minutes)
Time (minutes)
37
Speed Time Distance
60 Distance
Speed
Time
60 Distance
Time
Speed
Speed Time
Distance
60
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Speed and Distances

• Now calculate and label your speed from the
  distances of the legs you plotted on your chart.
• What is your speed from R N 2 outside Perkins
  Cove to R 10 Q R?
• 5.0 knots
• What is your speed from R 10 Q R to R N 4 on
  the Shark River channel?
• 6.5 knots
• What is your speed from R N 4 to RW OR on the
  Oyster River range?
• 4.0 knots

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Dead Reckoning

• Now calculate and label the DR positions on your
  course every hour on the hour..
• Plot 2 DR positions on the first leg at 1000 and
  1100
• Plot 1 DR positions on the second leg at 1200
• Plot 2 DR positions on the third leg at 1300 and
  1400

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Deck Log

• Now enter the fixes and DR positions into your
  deck log.
• Complete each compass entry and latitude and
  longitude
• Measure longitude and latitude to nearest
  parallel or meridian using dividers.

41
Relative Bearings

• The bearing of an object with respect to the
  fore-and-aft line of the ship is the relative
  bearing.
• Relative bearings can given in degrees or divided
  into 32 points (11.25 degrees per point).

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Example

• You should be able to translate from compass to
  relative bearings easily.
• Your ship is headed south (on a true course of
  180)
• You sight a ship on your starboard beam with a
  relative bearing of 090
• The true bearing of the sighted ship is
  270 (180 090 270)

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Relative Bearings
On the Starboard Beam
Dead Ahead
Dead Astern
On the Port Beam
44
Relative Bearings
45
Relative Bearings
On the Starboard Beam
Broad on the Starboard Bow
Broad on the Starboard Quarter
1
2
3
3
2
1
Dead Ahead
Dead Astern
Broad on the Port Bow
Broad on the Port Quarter
On the Port Beam
46
Relative Bearings
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Duties of a Lookout

• The rules of the road require a vessel to keep a
  proper lookout.
• In fog or at night, at least one lookout forward
  and one aft.
• A lookout is required even at anchor or backing
  out of a slip.

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Duties of a Lookout

• Must be vigilant, have no other duties, and be in
  a position to see and hear best.
• Must relate other evident pertinent information
  such as distance, course, and speed.
• In order to communicate an objects location, you
  must know and give the relative bearing of the
  object.

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Duties of a Lookout

• The lookout must be able to report accurately
• the range and bearing of any vessels
• aids to navigation, and
• objects sighted or heard.