How to apply the Compass Error

EARTH'S MAGNETIC FIELD

To understand how a marine magnetic compass is used it is necessary to understand the basic magnetic field of an ordinary magnet. The analogy of the sheet of iron filings used in the school physics lab is a good one. When a magnet is placed beneath the sheet of iron filings, they form themselves into a pattern illustrating the lines of magnetic force radiating from one pole to the other. The needle of a compass placed within this field will align itself with the lines of force that surround it. Now think of the world as the magnet. The lines of force radiate similarly from the poles and the needle of a compass placed within Earth's field will align itself with these lines of magnetic force. Since they emanate from north and south poles, the needle will align itself in the north-south direction. Unfortunately the magnetic poles are not situated in the same place as the true poles and thus an error occurs in the true reading of the compass. Therefore, before compass readings can be used for navigation they must be corrected for such errors.

VARIATION

This is the name given to the error caused by the difference in position between the true and magnetic poles. As its name denotes, it varies from place to place across the world, but it is accurately tabulated for the navigator's use. It can be described as follows: Variation is the error in the compass caused by the Earth's magnetism. It is always named E or W according to which direction the card is deflected away from true north.

DEVIATION

The second of the two errors which affect the magnetic compass, deviation, is caused by the magnetic influence of anything near the compass needle. Someone placing a metal knife alongside the binnacle, for example, will cause a deflection of the compass needle and result in deviation error. Steel in the construction of the ship, electric circuits, motors, and so on, can all affect the compass and create a deviation error. It would be safe to describe this error as follows: Deviation is the error in the compass caused by the ship's magnetism. It is always named E or W according to the direction the card is deflected from true north.

TO FIND VARIATION ERROR

This is easy. On every chart there are a number of compass roses as described in next paragraph below. In the centre of each compass rose is listed the variation for that area and the amount it is likely to change in one year, which is usually fairly small.

THE COMPASS ROSE

Since most navigation on a chart involves the use of the compass, a reproduction of a compass card is printed at strategic points across the face of every chart. These are termed compass roses and their positioning is such that there is always one close to hand no matter where on the chart the navigator is working. They are graduated in three-figure notations from 0° to 360° and contain details of the variations in force in that area.

TO FIND DEVIATION ERROR

This is not so easy. To begin with, new fittings, new stores or new equipment placed on the ship can add to the error of deviation. Providing the new products are not too magnetic and they are kept at least 1 metre away from the compass binnacle, they should not have too much effect, and this is worth remembering when fitting out a ship. The deviation can be affected with each change in the ship's direction, which creates another problem. Finally, the ship itself, particularly if it is of steel construction, will have become a magnet in its own right during its building period and, as can well be imagined, this will play havoc with the accuracy of the compass in the binnacle. The best way to find the deviation error is to engage a professional compass adjuster (Certified Compass Engineer) and have him attempt to eliminate the error or, if it cannot be eliminated, tabulate it on what is known as a deviation card.

THE DEVIATION CARD

When a ship is checked for deviation it must be checked on all headings since, as mentioned, deviation varies according to the course being steered. The Compass Engineer will swing the ship through the major compass points and determine the deviation on each point. The Compass Engineer will then list the deviation error on each heading on a deviation card. Thus the navigator can, by referring to this card, determine the deviation error on whatever course is being planned. Click for a larger Image [98 KB]

THE COMPASS CARD

For many years the traditional mariner's magnetic compass carried a card on which were ornately printed the cardinal points of the compass as well as three-figure notations. Modern magnetic compasses, however, have mostly done away with this cardinal system and have only three-figure notations (0° - 360°) inscribed on the edge of the card. The card commences at 0° (due north) and travels in one-degree units through 090° (due east), 180° (south), 270° (west) back to 360° or 0° at due north.

THE LUBBER LINE

The lubber line is the term given to the mark on the bowl of the magnetic compass which represents the centreline of the ship. Thus, when steering a course the card is swung until the figure representing the course lies against the lubber line. Digital compass do not have a lubber line but present the ship's heading as a three-figure readout.

COMPASS ERROR

As described earlier, every ship is affected by variation and deviation. A good magnetic compass engineer will eliminate the deviation if it is small, but, unfortunately, this is not always possible and both errors may be present. They are known collectively as compass error. Let's assume that we are sailing on an offshore reef when the weather turns thick and we are unable to see the shoreline. We know that the course (taken from the chart) back to port is 270° and we have a compass error of, say, 15°W. If we do not allow for the error, but steer the course of 270° on the compass, the error will push the ship 15° off course and it will wind up wrecked on the coastline to the south of the port entrance. By allowing for the compass error, the ship, although steering 15° to the north of the port entrance, will be pushed back 15° by the error and make a true course of 270° right into the harbour. Compass error is computed by adding or subtracting the variation to the deviation as follows: Like names add: unlike names subtract Thus: Variation 10°E Deviation 2°W = Compass Error 8°E. Variation 10°E Deviation 2°E = Compass Error 12°E.

APPLYING THE COMPASS ERROR

Since everything on the chart is drawn to true north, and everything done by compass is related to magnetic or compass north, it follows that between working on the chart and applying those workings in practice to the compass, the error must be allowed for. Applying the compass error can be done in a number of ways, notably by diagrams. But the easiest way to avoid confusion, and one which is absolutely foolproof, is to remember the jingle: Error east, compass least--error west, compass best An example is probably the best way to illustrate the use of this jingle: Variation 10°E True course on the chart 269°T Deviation 3°W Error 7°E (error east compass least) Error 7°E Course to steer by compass 262°C 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

SWINGING FOR COMPASS

It sometimes happens that the services of a compass engineer are not available to find the deviation. When this is the case the navigator must do it by a procedure known as swinging for compass. Below are the steps that should be followed: (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.

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