Compass Work

Question 1

Define Variation

Answer 1

Variation is the Angular Difference between the True Meridian and the Magnetic Meridian. Variation differs at different locations on the earth and undergoes an annual change. The error of variation is said to be in degrees East or West of True North.

Question 2

Define Deviation

Answer 2

Each vessel has its own magnetic field arising from the steel and iron used in its construction. This magnetic field will affect the magnetic compass and will cause an additional compass error called Deviation.

Question 3

What corrects for the ship’s hard iron component of error?

Answer 3

Fore & aft, athwartships, and vertical permanent magnets.

Question 4

What corrects for the ships soft iron component of error?

Answer 4

Soft iron spheres (Kelvin Spheres) aligned athwartships, and a Flinders bar (vertical soft iron rod) on the fore end of the compass.

Question 5

In addition to soft and hard iron, what else can affect the magnetic compass on board?

Answer 5

Siting of electrical equipment too close to the binnacle.

Question 6

What does a compass adjuster do?

Answer 6

They will swing the vessel to determine the deviations on all headings and arrange the correctors accordingly to minimise deviation. They will then produce a deviation card.

Question 7

What is the residual error after the compass has been ‘swung’ and what is it made up of?

Answer 7

Compass Error. Made up of Variation and Deviation

Question 8

How can you determine a True bearing?

Answer 8

• Two fixed landmarks are aligned, true bearing can be read from the chart.
• If observer is in a known, fixed location the true bearings of prominent landmarks can be read from the chart.
• Gyro compass can be used as long as gyro error is taken into account.
• The true bearing (Azimuth) of a celestial body can be determined.

Question 9

List the steps in finding the error of the compass

Answer 9

• Take a compass bearing of something.
• Calculate or find the true bearing.
• Find the difference between the Magnetic bearing and the True bearing, this is the error of the compass.
• Label it East if lower than True bearing or West if more.

Question 10

List further steps necessary to check the deviation card is correct for the ship’s head.

Answer 10

• At the same time as making an Compass Error observation note the ship’s head.
• From the chart calculate Variation.
• Remove this from the Compass Error to find the observed Deviation.
• Compare this with the Deviation for the ship’s head on the Deviation card