Compass errors arise from two sources: variation and deviation. Variation is the difference between magnetic and true north, while deviation is caused by nearby magnetic interference. Compass error is the sum of variation and deviation, representing the angle between true and compass north. Finding transit or gyro errors allows one to determine the compass error. Regular corrections are needed since errors change with location and vessel orientation.

1. COMPASS ERROR

2. INTENDED LEARNING OUTCOME
• At the end of the lesson, students should be able to learn the following:
• Knowing the Two parts of the magnetic compass error
• Knowing what is a Compass Error
• Knowing the difference between compass error and deviation
• Knowing the error if the magnetic field of a compass not being straight
• Finding the compass error
• Finding the transit error of a compass bearing
• Calculating the Gyro Compass

3. WHAT ARE THE TWO PARTS OF THE MAGNETIC COMPASS ERROR ?
- The Magnetic Compass Error is a combination of two separate and distinct components, namely variation
and deviation.
• Variation
- The needle in a compass is magnetized and freely suspended, and it aligns itself with the earth's magnetic
field. This difference between true north and magnetic north is known as variation. It's measured in degrees
and minutes (either east or west) and is shown on your chart in the compass rose.
• Deviation
- Magnetic deviation is the error induced in a compass by local magnetic fields, which must be allowed for,
along with magnetic declination, if accurate bearings are to be calculated. Deviation is the error in reading a
bearing from the compass caused by the magnetic influence of some nearby object, such as a metal post or
an engine. Most boats have some measure of deviation which should be minimised by careful mounting of the
compass. Deviation changes each time the boat alters course.

4. WHAT IS COMPASS ERROR?
• Compass Error is the difference between compass heading and true heading expressed as the
algebraic sum of variation and deviation. The Compass Error is a combination of two separate
and distinct components, namely variation and deviation. When influenced only by the earth's
magnetic field, a compass needle will point towards the earth's north magnetic pole. This is
magnetic north, and the angle between it and true north is called variation.
WHAT IS THE DIFFERENCE BETWEEN COMPASS ERROR AND
DEVIATION?
• Deviation can be East or West depending on whether Compass North lies to the East or West of
Magnetic North. While Compass Error is the algebraic sum of Variation and Deviation. It is
the difference between True North and Compass North.

5. WHAT IS THE ERROR IF THE MAGNETIC
NEEDLE OF A COMPASS NOT BEING STRAIGHT?
• Instrumental errors are those which arise due to the faulty adjustment of the instrument in the needle not
being perfectly straight, sluggish needle etc. Inaccurate centring, inaccurate levelling of compass box,
inaccurate bisection of signals etc. comes under personal errors.
HOW DO YOU FIND THE COMPASS ERROR?
• Compass Error - the algebraic sum of Variation and Deviation. It is the difference between True North and Compass
North. If Variation and Deviation are the same direction (lets say, both West), by adding together the angles of
Variation & Deviation, you will determine the Compass Error (also labeled West.) If the Variation and Deviation are in
different direction, assume the small number is a negative and add the 2 together, the direction will be that of the
larger angle.
Example:
Deviation = 1.5E
Variation = 5.0W
(-1.5) + 5.0 = 3.5
Compass Error = 3.5W

6. HOW DO YOU FIND THE TRANSIT ERROR OF A COMPASS BEARING?
• When two well charted objects are in transit a compass bearing is taken. The true bearing is taken from
the chart using parallel rulers. Comparison of the compass bearing and the true bearing gives
the compass error.

7. • Try checking the compass error using an azimuth circle on your compass and a Transit Bearing. A
more difficult one is using a pelorus for a Relative Bearing, then converting this to a Compass
Bearing.

8. • For Example:
If your gyro bearing by the use of Azimuth circle is 115.3°. Comparison of the Gyro bearing and the true
bearing gives the gyro error.
GB - 115.3°
TB - 115°
GE = 0.3° W
*ALWAYS REMEMBER THE RULE *
• Compass Best error West
• Compass Least error East

9. HOW IS GYRO COMPASS CALCULATED?
• The errors to which a gyrocompass is subject are speed error, latitude error, ballistic deflection error,
ballistic damping error, quadrantal error, and gimballing error. Additional errors may be introduced by a
malfunction or incorrect alignment with the centerline of the vessel. To solve gyro error we have to
subtract the gyro and true course. * The gyro error is named west because PGC is greater than the true
course. In case you forget how, always remember this that "Compass best, error west.
• For Example:
While your vessel is proceeding down the channel you notice a range of lights in line with your vessel’s
mast. If your vessel is on course of 001° per gyro compass and the charted value of the range of lights is
359 ° T, what is the gyro compass error?
Given:
Gyro Course = 001 °
True Course = 359 °

10. • What is asked?
- Gyro compass error
Solution:
To solve gyro error we have to subtract the gyro and true course .
PGC = 001° (add 360 ° to the PGC)
T/Co = - 359 °
G/E = 2 ° W
* The gyro is named west because PGC is greater than the true course. Compass Best, error west, Compass
least error East.

11. WHAT IS THE PURPOSE OF CORRECTING
COMPASS?
• The compass error should be checked each watch and on every new course to measure
the error of the gyrocompass, which is the angle the gyro north makes with the True north to
establish trends in deviation on different courses at varying latitudes.