NAVIGATION TRAINING
Section 4
Compass
Table of Contents
• Section 1
• Section 2
• Section 3
• Section 4
• Section 5
• Section 6
• Section 7
• Section 8
• Section 9
• Section 10
• Section 11
Types of Navigation
Terrestial Coordinates
Charts
Compass
Nautical Publications
Navigational Aids
Buoyage
Position Lines and Fixes
Tides
Currents
Weather
Compass
Types of Compasses
• Magnetic Compass
A compass that senses direction by
interaction between its own permanent
magnets and the earth’s magnetic field.
• Gyroscopic Compass
A electrical gyroscopic that is designed
to seek true north
Compass
• Compass Rose
Sometimes called a windrose, is a
figure on a compass, map, nautical
chart or monument used to display
the orientation of the cardinal
Compass
Compass
Directional Reference Systems
Relative Bearings (˚R) = bearings measured with reference
to the ship’s longitudinal axis
Magnetic Bearings (˚ M) = bearings measured with respect
to magnetic north.
True Bearings (˚ T) = bearings measured with respect to true
of geographic north.
Ship’s Head (or heading)= a special bearing denoting the
direction in which the ship is pointing.
Compass
True Bearings
000ºT
270ºT
090ºT
180º T
Compass
Magnetic Bearings
000ºM
Variation Easterly
270ºM
090ºM
180º M
Compass
Relative Bearings
270ºR
180ºR
000ºR
090º R
Compass
Relative Bearings
Port Beam
Right
Astern
Dead
Ahead
Starboard Beam
Compass
Magnetic Compass Error: Variation
• Variation is the angle between a magnetic line of force and a
geographic (true) meridian at any location on the earth.
• Variation exists because the earth’s magnetic and geographic
poles are not in the same location.
• Magnetic anomalies in the earth’s crust also contribute to
variation.
Compass
Magnetic
North Pole
True North
Pole
Notice that the two poles
aren’t together. The
magnetic compass points
to the magnetic pole, and
this gives us VARIATION.
Compass
Magnetic Compass Error: Variation
Variation also changes from year to year as the earth’s
magnetic poles tend to wander.
Variation is printed inside compass roses on all navigation
charts.
Always use the compass rose nearest your current Dead
Reckoning position.
Variation changes as an observer moves along the globe.
However, if a ship moves in such a way that the meridians
remained constant, it would be moving along an isogonic line
- a line along which variation remains constant.
Compass
Magnetic Compass Error: Variation
The amount that the Variation changes annually is called the
Annual Change.
The amount of Annual Change is to be found on every
compass rose on the chart, next to the Variation, and is
normally expressed as 004°W 1995 (8’E).
To calculate change in 2008, multiply 8’E by 13 (years from
1995) 104’ or 1.75°E
Apply 1.75°E to Variation of 004°W = 2.25°W
Compass
Magnetic Compass Error: Variation
Compass
Magnetic Compass Error: Deviation
•This is the angle between the magnetic meridian and the
north line on the compass card.
•Deviation is caused by the interaction of the ship’s metallic
structure, electrical systems, metallic objects (such as a cell
phone left close to the compass) with the earth’s magnetic
field.
Compass
Deviation
A ship’s compass also must
deal with magnetic forces
from the ship itself, e.g.
magnets, electrical cabling.
The sum total of these forces
pulls the compass slightly
away from magnetic north,
producing DEVIATION.
Compass
Deviation
Deviation will change in size,
dependant upon the course of
the vessel.
Swinging of the ship and
proper correction using soft
and/or permanent magnets ,
deviation can be minimized.
Compass
Compass Conversions
Compass to True
1. C D M V T (AE)
Can Dead Men Vote Twice (at elections)?
2. C A D E T
Compass Add East True
Compass
Compass Conversions
Convert Compass Courses to True Courses - this also applies
to bearings, using the deviation for the vessel’s head.
Compass Course
Deviation
Magnetic Course
Variation
True Course
145˚ C
2˚W
143˚M
22˚E
165˚T
Compass
Compass Conversions
Converting True to Compass
T V M D C (AW)
True Virgins Make Dull Companions (At Weddings)
Compass
Compass Conversions
Convert True Course to Compass Course - this also applies to
bearings, using the deviation for the vessel’s head.
True Course
Variation
Magnetic Course
Deviation
Compass Course
165˚ T
22˚E
143˚M
2˚W
145˚C