How a Compass Works
A marine magnetic compass works the same way as an ordinary magnet. In the basics of a magnet field, when a magnet is aligned beneath a sheet and iron filling placed on top of the sheets, the fillings will align themselves in a pattern in a manner to illustrate the lines of magnetic force radiating from one pole to the other. The same case will apply to the needle of a compass.
The earth too has magnetic forces emanating from the south and north poles and thus the needle of a magnetic compass will too align itself in the north-south direction. However, you need to note that the magnetic poles of the compass are usually not situated in the same place as the earth’s poles and this may results errors when interpreting the compass. Therefore there is a need to correct the errors.
An error which results due to the differences in position of the earth poles and the magnetic poles is called the compass variation. The error varies from one place to the other.
This is a compass error which might result in case there is a different magnetic influence near the compass at the time of reading it. In most ships, this error is common because they are made of steel and anything metallic could have magnetic influence.
Finding the Variation Error
Variation errors can be found on nautical charts which are found in every ports, harbors, ship etc. These charts differ because the error varies from place to place. On the chart, there are several compass roses and at the center of each rose there is information about the error of that region.
Finding the Deviation Error
It is comparatively hard to find the deviation errors because any new equipment or implement that is placed on a ship results to additional deviation errors. Therefore such equipment if they are metallic and could possess some magnetic force should be kept away from the compass to reduce chances of error.
Still another problem arises in case of changing the cause of navigation e.g. due to storms or bad weather because that can too result to additional errors. The same applies to steel made ships which contribute much to the deviation. Therefore the best way to reduce chances of this error is to have a professional compass adjuster make the necessary adjustments. If the error is not completely adjusted then you need to tabulate the data of deviation on each part of the ship as well as the course of navigation on a deviation card
Applying Compass Error
As we have noted, it is impossible to completely eradicate variation and deviation errors (collectively known as compass error). However, since everything on the nautical chart is drawn true to the north, and the magnetic compass has its needle pointing north too, all you have to do is to allow for an error margin.
The best approach which is foolproof is to remember the adage ‘error east, compass least- error west compass best.
Variation 10 degrees east- true course on the chart 296 degrees
Deviation 3 degrees west- error 7degeress east (error east compass least)
Therefore error 7 degrees east- course to steer by compass is 262 degree
Variation 10°E True course from chart 269°T
Deviation 17°W Error 7°W (error west compass best)
Error 7°W Course to steer by compass 276°C
In case you don’t avail the services of a compass adjuster before navigating, the rules below could help you steer the ship correctly
1) Locate two transit objects (objects in line) and determine their true bearing on the chart.
(2) Secure the ship at anchor so that these transits are exactly aligned. Swing the ship’s head until it is pointing due north.
(3) Read off the transit bearing on the compass and apply the variation.
(4) The difference between this result and the true transit bearing is the deviation on this heading. If the true bearing is greater, the deviation is named east; if it is the lesser, the deviation is named west.
(5) Repeat the procedure taking transit bearings on each of the cardinal points.
(6) From the results, make up a deviation card.