郑嘉颖吐槽颖儿:Navis.gr - How to apply the Compass Error

Howto apply the Compass Error
EARTH'S MAGNETIC FIELD

To understand how a marinemagnetic compass is used it is necessary to understandthe basic magnetic field of an ordinary magnet. Theanalogy of the sheet of iron filings used in the schoolphysics lab is a good one. When a magnet is placedbeneath the sheet of iron filings, they form themselvesinto a pattern illustrating the lines of magnetic forceradiating from one pole to the other. The needle of acompass placed within this field will align itself withthe lines of force that surround it.

Now think of the world as the magnet. The lines of forceradiate similarly from the poles and the needle of acompass placed within Earth's field will align itselfwith these lines of magnetic force. Since they emanatefrom north and south poles, the needle will align itselfin the north-south direction. Unfortunately the magneticpoles are not situated in the same place as the truepoles and thus an error occurs in the true reading of thecompass. Therefore, before compass readings can be usedfor navigation they must be corrected for such errors.
VARIATION

This is the name given to the error caused by thedifference in position between the true and magneticpoles. As its name denotes, it varies from place to placeacross the world, but it is accurately tabulated for thenavigator's use. It can be described as follows:

Variation is the error in the compass caused by theEarth's magnetism. It is always named E or W according towhich direction the card is deflected away from truenorth.

DEVIATION

The second of the two errors which affect themagnetic compass, deviation, is caused by the magneticinfluence of anything near the compass needle. Someoneplacing a metal knife alongside the binnacle, forexample, will cause a deflection of the compass needleand result in deviation error. Steel in the constructionof the ship, electric circuits, motors, and so on, canall affect the compass and create a deviation error. Itwould be safe to describe this error as follows:

Deviation is the error in the compass caused by theship's magnetism. It is always named E or W according tothe direction the card is deflected from true north.

TO FIND VARIATION ERROR

This is easy. Onevery chart there are a numberof compass roses as described in next paragraph below. Inthe centre of each compass rose is listed the variationfor that area and the amount it is likely to change inone year, which is usually fairly small.

THE COMPASS ROSE

Sincemost navigation on a chartinvolves the use of the compass, a reproduction of acompass card is printed at strategic points across theface of every chart. These are termed compass roses andtheir positioning is such that there is always one closeto hand no matter where on the chart the navigator isworking. They are graduated in three-figure notationsfrom 0° to 360° and contain details of the variationsin force in that area.

TO FIND DEVIATION ERROR

This is not so easy. To begin with, new fittings, newstores or new equipment placed on the ship can add to theerror of deviation. Providing the new products are nottoo magnetic and they are kept at least 1 metre away fromthe compass binnacle, they should not have too mucheffect, and this is worth remembering when fitting out aship. The deviation can be affected with each change inthe ship's direction, which creates another problem.Finally, the ship itself, particularly if it is of steelconstruction, will have become a magnet in its own rightduring its building period and, as can well be imagined,this will play havoc with the accuracy of the compass inthe binnacle.
The best way to find the deviation error is to engage aprofessional compass adjuster (Certified CompassEngineer) and have him attempt to eliminate the error or,if it cannot be eliminated, tabulate it on what is knownas a deviation card.

THE DEVIATION CARD

When a ship is checked for deviation it must bechecked on all headings since, as mentioned, deviationvaries according to the course being steered. The CompassEngineer will swing the ship through the major compasspoints and determine the deviation on each point. TheCompass Engineer will then list the deviation error oneach heading on a deviation card. Thus the navigator can,by referring to this card, determine the deviation erroron whatever course is being planned.

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THE COMPASS CARD

For many years the traditional mariner's magneticcompass carried a card on which were ornately printed thecardinal points of the compass as well as three-figurenotations. Modern magnetic compasses, however, havemostly done away with this cardinal system and have onlythree-figure notations (0° - 360°) inscribed on theedge of the card.

The card commences at 0° (due north) and travels inone-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 thebowl of the magnetic compass which represents thecentreline of the ship. Thus, when steering a course thecard is swung until the figure representing the courselies against the lubber line. Digital compass do not havea lubber line but present the ship's heading as athree-figure readout.

COMPASS ERROR

As described earlier, every ship is affected byvariation and deviation. A good magnetic compass engineerwill eliminate the deviation if it is small, but,unfortunately, this is not always possible and botherrors may be present. They are known collectively ascompass error.

Let's assume that we are sailing on an offshore reef whenthe weather turns thick and we are unable to see theshoreline. 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 steerthe course of 270° on the compass, the error will pushthe ship 15° off course and it will wind up wrecked onthe coastline to the south of the port entrance.

By allowing for the compass error, the ship, althoughsteering 15° to the north of the port entrance, will bepushed back 15° by the error and make a true course of270° right into the harbour.

Compass error is computed by adding or subtracting thevariation to the deviation as follows:

Like names add: unlike names subtract

Thus: Variation 10°E Deviation 2°W = Compass Error8°E.
Variation 10°E Deviation 2°E = Compass Error 12°E.

APPLYING THE COMPASS ERROR

Since everything on the chartis drawn to true north, and everything done by compass isrelated to magnetic or compass north, it follows thatbetween working on the chart and applying those workingsin practice to the compass, the error must be allowedfor.

Applying the compass error can be done in a number ofways, notably by diagrams. But the easiest way to avoidconfusion, and one which is absolutely foolproof, is toremember the jingle:

Error east, compass least--error west, compass best

An example is probably the best way to illustrate the useof 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 compassengineer are not available to find the deviation. Whenthis is the case the navigator must do it by a procedureknown as swinging for compass. Below are the steps thatshould be followed:

(1) Locate two transit objects (objects in line) anddetermine their true bearing on the chart.
(2) Secure the ship at anchor so that these transits areexactly aligned. Swing the ship's head until it ispointing due north.
(3) Read off the transit bearing on the compass and applythe variation.
(4) The difference between this result and the truetransit bearing is the deviation on this heading. If thetrue bearing is greater, the deviation is named east; ifit is the lesser, the deviation is named west.
(5) Repeat the procedure taking transit bearings on eachof the cardinal points.
(6) From the results, make up a deviation card.