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Compass Work

Compass Corrections

Definitions

True North: This refers to the geographical North Pole. This is a physical pole since the axis of the earth passes through the same. All charts are aligned to this pole and the co-ordinate system refers to this as the North Pole.

Magnetic North: This is the south-seeking pole of the earth when considered as a giant magnet. All magnetic compasses point to this pole as North. The physical and the magnetic north pole do not coincide. The magnetic pole shifts over time as the earth cools down and also due to other various reasons. The physical pole remains stationary.

Compass North: This takes into account both variation and deviation experienced by the compass while pointing the direction of North. It is not possible to have two ships compass point at the same direction as North.

Magnetic Course:

The angle between the magnetic meridian and the direction of the ship’s head. It defines the direction of the ship’s head relative to “Magnetic North”. The difference between the two is the Variation.

True Course: After allowing for Deviation and Variation to the Magnetic Course/bearing.

Compass Course: The angle between the compass needle and the direction of the ship’s head. It defines the direction of the ship’s head relative to “Compass North”. The compass course is indicated by the position of the ‘lubber’s line’ relative to the compass card. Both deviation and variation are involved in this correction.

Finding deviation and variation from tables and charts

Variation may be found from variation charts as well as from that printed on the compass rose on any navigational chart.

Calculating true course from compass course

Given: Compass Course - 110° and on this course the deviation is 3°E, the chart shows a Variation of 9°W, to find the True Course.

We may combine the two errors - 9°W and 3°E, this becomes a combined error of 6°W. Or we may say that the compass error is 6°W.

Now using the quote:

‘Error West Compass Best – Error East Compass Least’

We see that the Error is West so the compass would be the best or the greater than the True.

So, the True course would be less than the compass course by 6°. The True course therefore would be - 104°

If we do this step by step then:

Compass Course          - 110°

Deviation                      - 3°E

Magnetic Course          - 113° (Error East Compass Least, so True in this case Magnetic is more so add)

Variation                      - 9°W

True Course                 - 104° (Error West Compass Best, so True is less, so subtract)

Calculating compass course from true course

Given: True Course - 110° and on this course the deviation is 3°E, the chart shows a Variation of 9°W, to find the Compass Course.

We may combine the two errors - 9°W and 3°E, this becomes a combined error of 6°W. Or we may say that the compass error is 6°W.

Now using the quote:

‘Error West Compass Best – Error East Compass Least’

We see that the Error is West so the compass would be the best or the greater than the True.

So, the Compass course would be greater than the True course by 6°. The Compass course therefore would be - 116°

If we do this step by step then:

True Course                - 110°

Variation                      - 9°W

Magnetic Course          - 119° (Error West Compass Best, so Compass in this case Magnetic is more so add)

Deviation                      - 3°E

Compass Course                      - 116° (Error East Compass Least, so Compass is less, so subtract)

Using a transit bearing to find error

Transit bearings are usually taken within Pilotage waters or in very near coastal waters. Two prominent marks are selected – generally a lighthouse and another beacon or a building (should be marked on the chart). A line is drawn between the two and extended to cut the own vessel course line at a future time.

It thus becomes obvious that the transit line (the extended line) should cut the course line.

Once this is done the transit line is read off from the compass rose and the same is written on the chart next to the line.

An estimated time is also written down of approaching this point where the transit line would be cutting the course line.

A few minutes prior to the time the two objects are sighted through the azimuth mirror and at the time of actual transit the bearing is noted. This is then compared with that which was read off the compass rose. This gives the error of the compass.

While entering port the pilot generally looks up at the leading lights which are aligned at a certain bearing and confidently tells the Master that the compass has an error or not and of the error amount.

Applying compass error to the ship's head and compass bearings to convert to true

Ships course Correction:

Given: Compass Course - 120° and on this course the deviation is 4°E, the chart shows a Variation of 9°W, to find the True Course.

We may combine the two errors - 9°W and 4°E, this becomes a combined error of 5°W. Or we may say that the compass error is 5°W.

Now using the quote:

‘Error West Compass Best – Error East Compass Least’

We see that the Error is West so the compass would be the best or the greater than the True.

So, the True course would be less than the compass course by 5°. The True course therefore would be - 115°

If we do this step by step then:

Compass Course          - 120°

Deviation                      - 4°E

Magnetic Course          - 124° (Error East Compass Least, so True in this case Magnetic is more so add)

Variation                      - 9°W

True Course                 - 115° (Error West Compass Best, so True is less, so subtract)

Observed Bearing Correction:

Given: Compass Course - 110° and on this course the deviation is 3°E, the chart shows a Variation of 9°W, Bearing of a light - 145°, to find the True Bearing.

We may combine the two errors - 9°W and 3°E, this becomes a combined error of 6°W. Or we may say that the compass error is 6°W.

Now using the quote:

‘Error West Compass Best – Error East Compass Least’

We see that the Error is West so the compass bearing would be the best or the greater than the True bearing.

So, the True bearing would be less than the compass bearing by 6°. The True bearing therefore would be - 139°

If we do this step by step then:

Compass bearing          - 145°

Deviation                      - 3°E

Magnetic bearing          - 148° (Error East Compass Least, so True in this case Magnetic is more so add)

Variation                      - 9°W

True bearing                 - 139° (Error West Compass Best, so True is less, so subtract)

REMEMBER THE DEVIATION IS OF THE COMPASS IS ON A PARTICULAR SHIPS HEADING – (THE MAGNETIC LINES OF THE EARTH CUT THE SHIP DIFFERENTLY ON DIFERENT HEADINGS) – AS SUCH DO NOT LOOK UP THE DEVIATION ON THE BASIS OF THE BEARING BUT LOOK UP FOR THE SHIPS HEADING.