Diederik Willemsen www.sailingissues.com
China compasses have been in use since the Han dynastie (2nd
century BCE to 2nd century CE) when they were referred to as
'south-pointers'. However at first these magnets were only used for
geomancy much like in the art of Feng Shui. Eventually, during the Sung
dynasty (1000 CE) many trading ships were then able to sail as far as
Saudi Arabia using compasses for marine navigation. Between 1405 and 1433,
Emperor Chu Ti's Treasure Fleet of the Dragon Throne ruled the entire
South Pacific and the Indian Ocean, a territory that ranges from Korea and
Japan to the Eastern coast of Africa.
At this time Western mariners were
still rather ignorant of the navigational use of the magnet. Petrus
Perigrinus van Maricourt
wrote a first treatise on the magnet itself: "De Magnete" (1269). And
though its nautical use was already mentioned in 1187 by the English monk
Alexander Neckham, the use onboard only came about around the 13th and
14th century in the Mediterranean Sea. Much later, in 1545, Pedro de
Medina (Sevilla 1493-1567) wrote the Spanish standard work "Arte de
Navegar" on marine compass navigation. This masterpiece was first
translated in Dutch (1580) and was -O Irony- used by Jacob van Heemskerk
when the Dutch destroyed the Spanish fleet near Gibraltar in 1607. The
drawback was of course Van Heemskerk's own death during this victory.
In the fin-de-siècle of the sixteenth
century mariners believed that the North magnetic pole coincided with the
North geographic pole. Any suggestion otherwise had been denied by Pedro
de Medina. Magnetic observations made by explorers in subsequent decades
showed however that these suggestions were true. But it took until the
early nineteenth century, to pinpoint the magnetic north pole somewhere in
Arctic Canada (78° N , 104° W). From then on the angle between the true
North and the
Magnetic North could be precisely corrected for. This correction
angle is called Magnetic Variation or
declination. It is believed that the Earth's magnetic field is produced by
electrical currents that originate in the hot, liquid, outer core of the
rotating Earth. The flow of electric currents in this core is continually
changing, so the magnetic field produced by those currents also changes.
This means that at the surface of the Earth, both the strength and
direction of the magnetic field will vary over the years. This gradual
change is called the Secular Variation of the
magnetic field. Therefore, variation changes not only with the location of
a vessel on the earth but also varies in time.
Correcting for Variation
The correction for magnetic variation is
shown for your location on your current navigation chart's compass rose.
Take for example a variation of 2° 50' E in 1998. In 2000, this variation
is estimated to be 2° 54', almost 3° East. This means that if we sail 90°
on the chart (your true course), the compass would read 87°.
To convert your true course into a
compass course we need first assign a "-" to a
Western and a "+" to a Eastern variation. From the following equation you
will see that this makes sense:
87° cc + 3° var = 90° tc
, in which 'cc' and
'tc' stand for 'compass
course' and 'true course', respectively.
We can use the same equation to convert a compass course into a true
course. If we steered a compass course of 225° for a while, we have to
plot this as a true course of 228° in the chart.
Magnetic deviation is the second
correctable error. The deviation error is caused by magnetic forces within
your particular boat. Pieces of metal, such as an engine or an anchor, can
cause magnetic forces. And also stereo and other electric equipment or
wiring , if too close to the compass, introduce large errors in compass
heading. Furthermore, the deviation changes with the ship's heading,
resulting in a deviation table as shown below. The vertical axis states
the correction in degrees West or East, where East is of course positive.
The horizontal axis states the ship's heading in degrees divided by ten.
Thus, when you sail a compass course of 220°, the deviation is 4° W.
When a compass is newly installed it often shows larger deviations than
this and needs compensation by carefully placing small magnets around the
compass. It is the remaining error that is shown in your deviation table.
You can check your table every now and then by placing your boat in the
line of a pair of
leading lights and turning her 360 degrees.
Correcting for both Deviation and
Converting a compass course into a true
course, we can still use our equation but we need to add the correction
cc + var + dev = tc
The compass course is 330°, the deviation is +3° (table) and the
variation is +3° (chart);
330° cc + 3° var + 3° dev = ?° tc
giving a true course of 336° which we can plot in our chart
The compass course is 220°, the deviation is -4° (table) and the
variation is still +3° (chart).
220° cc + 3° var + -4° dev = ?° tc
giving a true course of 219° which we can plot in our chart.
Converting a true course into a compass
course is a little less straight forward, but it is still done with the
The true course from the chart is 305° and the variation is +3° (chart),
yet we don't know the deviation;
?° cc + 3° var + ?° dev = 305° tc
Luckily, rewritten this reads:
305° tc - + 3° var = cc + dev = 302°
In plain English: the difference between the true course and the
variation (305 - + 3) = 302 should also be the summation of the compass
course and the deviation. So, we can tell our helmsperson to steer 300°,
since with a cc of 300° we have a deviation of +2° (As can be deduced
from the deviation table above).
The true course from the chart is 150° and we have a Western variation
of 7 degrees (-7°). We will use the rewritten equation to get:
150° tc - - 7° var = cc + dev = 157°
From the deviation table we find a compass course of 160° with a
deviation of -3°. Voilá!
course (mc) is the heading after magnetic variation has been
considered, but without compensation for magnetic deviation. This means
that we are dealing with the rewritten equation from above:
tc - var = cc + dev = mc.
Magnetic courses are used for two reasons. Firstly, the magnetic course is
used to convert a true course into a compass course like we saw in the
last paragraph. Secondly, on boats with more than one compass more
deviation tables are in use; hence only a magnetic or true course is
plotted in the chart. To summarise, we have three types of 'North' (True,
Magnetic and Compass north) like we have three types of courses (tc, mc
and cc). All these are related by deviation and variation.
The angle between the magnetic North pole and the geographic North pole.
Also called the magnetic declination.
Variation: The change of magnetic declination
in time with respect to both strength and direction of its magnetic
(-) , East
Western variations or deviations are designated with a negative sign by
convention due to the compass card's clock wise direction.
The error in compass heading caused by electric magnetic currents and or
Table: A table containing deviations in
degrees versus the ship's heading (compass course) in degrees. Usually
plotted in a graph.
Course plotted in the chart i.e. course over the ground or 'course made
good'. The course corrected for compass errors.
The course (ship's heading) without the correction for compass errors.
cc + var + dev
= tc: This equation shows the connection
between the compass course, its errors and the true course. It can also
be read as: tc - var = cc + dev.