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Lesson 3 True course | Coastal Navigation

The compass

There are 3 kinds of north directions: compass north = Nc, magnetic north Nm and true north = Nt (or Ng, which stands for geographic north). Compass north is the direction the compass needle points to. The magnetic north is where the compass would point to if it was not influenced by magnetism of steel parts onboard. The true north is the direction to the North Pole. The compass does not always point precisely to the North Pole. This is due to the compass errors: variation and deviation. Together we call them the two-part compass error. If the error is counter clockwise, so from the north to the west, the error is negative. Clockwise, from the north to the east is positive.

The 32 compass points can be found in the table below.

N

Noord

11¼°

NbE

North by east

22½°

NNE

North-northeast

33¾°

NEbN

Northeast by north

45°

NE

Northeast

56¼°

NEbE

Northeast by east

67½°

ENE

East-northeast

78¾°

EbN

East by north

90°

E

East

101¼°

EbS

East by south

112½°

ESE

East-southeast

123¾°

SEbE

Southeast by east

135°

SE

Southeast

146¼°

SEbS

Southeast by south

157½°

SSE

South-southeast

168¾°

SbE

South by east

180°

S

South

191¼°

SbW

South by west

202½°

SSW

South-southwest

213¾°

SWbS

Southwest by south

225°

SW

Southwest

236¼°

SWbW

Southwest by west

247½°

WSW

West-southwest

258¾°

WbS

West by south

270°

W

West

281¼°

WbN

West by north

292½°

WNW

West-northwest

303¾°

NWbW

Northwest by west

315°

NW

Northwest

326¼°

NWbN

Northwest by north

337½°

NNW

North-northwest

348¾°

NbW

North by west

360°

N

North

Variation

Variation is the result of the fact that the magnetic fields that run from the south pole to the north pole over the earth are not straight, but serpentine shape. As a result, every compass in one place points a bit too much to the West and in the other place a little too much to the East. We get the variation from the nearest variation compass on the nautical chart.

variatieroos

The magnetic fields move and so the variation also changes over time per location. If you find 10° W 2002 (8'E) in the variation compass in the nautical chart, that means that the variation was 10 degrees West in 2002 and moves 8 minutes per year to the East. This information you will find in nautical charts, but never in the detailed coastal sea charts, like the Dutch 1800 series, because they have to be renewed again every year, because those nautical charts change a lot.

If we would like to calculate the variation in 2013, we calculate this as follows:
2013 compared to 2002 is 11 years later.
11 years x +8 minutes = +88 minutes, or + 1 ° 28 '.
-10 + 1 ° 28' = -8 ° 32 '.
The variation in 2013 is therefore 8 ° 32'W.
Note: 8 ° 32'W is rounded up to whole degrees like this: 9 ° W, because 32' is more than half of 60', so we round up.

Deviation

Deviation is created by magnetism on board. For example, the engine affects the steering compass. Because the compass needle always points to the north and the magnetism moves around the compass needle, the deviation is dependent on the course. In the deviation table, you can find the deviation that belongs to a certain compass course. Below is an example of a deviation table:

Compass course

Deviation

Compass course

Deviation 

-4

202,5°  +2

22½°

-2

225°  0

45°

 0

247,5°  -2

67½°

+2

270°  -4

90°

+4

292,5°  -5

112½°

+5

315°  -6

135°

+6

337,5°  -5

157½°

+5

360°  -4

180°

+4 

   

Also hand bearing compasses are influenced by deviation, so use hand bearing compasses as far away from any magnetic parts onboard.

When we use a hand bearing compass, we assume that the deviation is 0, because we do not have a deviation tabel belonging to the hand bearing compasses, as it has no fixed place onboard.

A deviation table gives the deviation per compass point. Sometimes you need to take the average between two compass points.

Heeling Error

The heeling of a ship affects the compass needle. Magnetism that is normally positioned right under the compass needle doesn't cause an error to the compass needle. However, when the ship starts heeling that magnetism moves sideways and starts to cause an error. If the heeling doubles, the horizontal force is also doubled and with it the error. The heeling error is at maximum on north - south courses. At east - west courses it is 0, because the force of the magnetism then pulls the needle in north or south direction and gives no error to the compass needle.

To calculate the heeling error, you first have to convert the true course to the compass course, on which you need to calculate the heeling error. Then you determine the heeling error on the compass course 0 degrees. Then the heading factor is determined by reading the cosine graph or by using your calculator (type the degrees and press COS). You then determine the heeling factor, the heeling error is directly proportional to the heeling, up to 20 degrees of heeling. Thus, 2 x as much heeling means 2 x as much heeling error and a heeling error of e.g. 4 degrees at 10 degrees heeling over starboard means a heeling error of -4 degrees at a heeling of 10 degrees over the port side. Finally, you calculate the heeling error by multiplying the heeling error on a compass course of 0 degrees with the heading factor and the heeling factor.

Converting a compass course into a true course and the other way around

Use this mnemonic to remember the formula to convert a compass course into a true course or the other way around:

True virgins make dull company.
TVMDC: True, Variation, Magnetic, Deviation, Compass

Put this calculation one under the other, so vertical, like here under, and no matter what... leave the compass course on top:

CC compass course
DEV Deviation                          +
MC magnetic course
VAR Variation                            +
TC true course

Fill in as much as possible and calculate what is missing.

Converting a true course into a compass course

CC                          ?
DEV                        -2+
MC                         98

Ask yourself “What plus -2 is 98?
Answer = 100

We use the magnetic course to lookup the deviation in the deviation tabel, while the deviation in the tabel is given for compass courses. However, we don't know the compass course jet.

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Questions and answers

Question 1: What is the variation in 2002 if this is written in the variation compass in the nautical chart: 4° W 1983 (8’E)?

Vraag 2: How many compass points there are in a compass?

Question 3: Compass course 45°, var 3°W, dev -5° What will be the true course?

Question 4: The true course is 0°, var 3° west, dev -1° What is the compass course?

Question 5: What is the difference between the compass course and the magnetic course?

Question 6: What is the deviation in the above deviation table at the compass course of 280 degrees?

Question 7: You are sailing on the leading lights of Enkhuizen on a compass course of 36°. You see in the nautical chart that the true bearing on those leading lights also are 36°. In the variation compass there is a variation of –2°. What is the deviation?

Question 8: The true course is 185. The variation is +3. You can lookup the deviation in the table above. What will be the compass course you need to steer?

Question 9: On which course the heeling error is largest?

Question 10: Deviation is the difference between the:

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