**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.