Chapter - 10

Compass Errors

Chapter Ix was devoted to the preliminary study of the compass, the learning of the various names for the thirty-two points of the compass and the names of all of the quarter points. We now come to a further consideration of the compass, and principally to its many errors and the application of variation and deviation as applying to small boat practice.

As we endeavored to emphasize in the last chapter, there is nothing mysterious about the compass or any of its actions. Therefore, the motor boatman should not consider his compass an inconquerable foe, but quite to the contrary, he should learn to respect and believe in what his compass tells him. More disasters have occurred due to the fact that the mariner believed that he knew more than his compass, than from any other cause. It is true that the compass is subject to certain errors to be explained in this chapter but these errors may be readily determined and allowed for. Therefore, every motor boatman should study his compass, learn to believe in it and make use of it on every possible occasion.

Kinds Of Courses

The three methods by which bearings or courses may be expressed are:

(a) True, when they refer to the North geographical pole.

(b) Magnetic, when they refer to the North magnetic pole. To change magnetic courses to true courses, the former must be corrected for variation.

(c) Compass, when they refer to the particular boat's compass on each particular heading and must be corrected for the deviation on that heading for conversion to the magnetic course and for both deviation and variation for conversion to true hearings or true courses.

Variation is the angle between two lines drawn the ship's position. One is a magnetic meridian through the magnetic north pole MN, the other a Ml through the geographic (true) north pole. If the north point of the compass cord is drawn to the west or left of geographic north fas shown), variation is westerly. If it ¡s drawn to the east or right of geographic north, variation is easterly. When changing from magnetic to true courses or vice verso allow for variation only. When changing from true to compass courses, or vice versa, allow for both variation and deviation passing.
 
In addition to variation, there may be another error of the compass, caused by various magnetic substances on the boat itself attracting or repelling certain points of the compass. This error is known as deviation. When the north pole of the compass is turned to the left, or to the west from its normal position, as illustrated above, the deviation is said to be westerly. If the north pole is pulled toward the east, or to the right, the deviation is said to be easterly. Obviously, the deviation ¡s different for each-particular boat, and furthermore, is different for each particular heading of the boat

In the diagram at the right some liberties ore taken, for the sake of simplification, in illustrating the behavior of the magnetic compass. Its needle tends to align itself, not with the geographical (true) converging at the north pole, but rather with the magnetic lines surrounding the north pole in the general area indicated at MN. These lines of force are neither straight nor constant, varying according to the locality and the particular year. Aligning itself with magnetic median, the compass (disregarding deviation) determines magnetic direction, courses, bearings and headings, as distinguished from the true directions established by reference to the true geographical meridians. The angular" difference between a true meridian and a magnetic meridian is the variation at that particular locality. At A, true north lies on the boat's course, while her compass (disregarding deviation) points, roughly, to MN. Similarly, at D, C, and D, the compass tends to align itself with the magnetic meridian of the locality. At A and B, variation is westerly; at D it is easterly; at points near C it approaches zero. There is a slight yearly change in variation at ony particular place (as indicated on the chart). This annual change is small and can be considered nearly constant although it does vary slightly at times

Variation

If you were on your boat at the position marked A, and your boat was heading as indicated by the dotted line, she would be heading true North, but the compass would be pointing in a decidedly different direction, indicated by the dotted line from A with a point marked MN. In other words, your boat would be headed true North, but the magnetic heading would be quite different. This angular difference between the true North and the magnetic North is known as the variation of the compass, shown in the figure by the angle between North Pole, A, and MN.

This variation is not constant; it is different with every change in geographical location. If your position is at B, your boat is still heading true North, and your compass towards MN. The angle between true North and magnetic North at B is different and smaller than when at A. In other words, the variation at B is less than at A. In both cases magnetic North has been to the West, or to the left, of true North, which makes the variation westerly.

Now consider your position at C. In this case your boat is heading toward true North, and also toward magnetic North. If the magnetic meridian coincides exactly with the true direction of geographic north, there is no variation. (The magnetic meridians are never perfectly straight lines.)

At D we again have a variation, but in this case magnetic North is to the East, or to the right of, true North, and we have an easterly variation. At E, the boat is heading toward true North, but going away from magnetic North. In such a case, while the boat is heading North, the compass is pointing South, and we have 180 degrees variation.

Change In Location

As has just been mentioned, the variation of the compass is different for every geographical location. In the vicinity of New York City the variation is about 13 degrees westerly; around Portland, Me., it is about 20 degrees westerly. As we go West the variation becomes less and less until in the vicinity of Lake Superior we have zero variation. Farther West than this the variation becomes easterly, and increases in magnitude.

Determining Variation

One may now ask how to determine what this variation is. This is a relatively simple matter, as on every one of our Go eminent charts this information is given. The illustration on this page shows what is known as the compass rose on the chart, several of which are printed on every one. The note in the center of this rose, "Variation 17 degrees 45 minutes West in 1921," gives the information in regard to variation at the particular location where this rose is printed. From the statement directly below, "Annual increase 4 minutes," it will be recognized that variation is not a constant quantity, but is increasing or diminishing all the time. To calculate what the variation is in 1922, we simply must add 4 minutes to the variation as noted.

At the left illustrates a compass rose showing a westerly deviation of about one and one-half points. By comparing this compass rose with the one shown at right, Indicating an easterly variation, one will be able to readily distinguish between the two. To determine the amount and direction of variation it is only necessary to refer to the compass rose on the various charts. This is invariably given in the center of every compass rose. It should also be noted that the variation of the compass varies in the amount for a particular locality from year to year. This variation in variation is also noted on the compass rose. It will be noticed that the annual increase is four minutes. Therefore, to determine the amount of variation for any particular year it is only necessary to do a little calculating

At the right shows the compass rose on the chart showing an easterly variation of 23 degrees, which is equivalent to approximately two points. It will be remembered that by easterly variation we mean that the magnetic north is to the right or eastward of the true north. One will notice by observing this compass rose that the rose consists of an outer and inner circle each of which is divided into a certain number of divisions. It will also be noted that the divisions do not correspond. The outer circle indicates true directions and the inner circle indicates magnetic directions. When determining true courses one will have to refer to the outer set of divisions yet, while working with magnetic courses or directions, he must use the directions indicated on the inner circle

On this compass rose there are an inner and an outer set of divisions, the inner one being in points and quarter points, and the outer in degrees. The two do not correspond—that is the North magnetic division is not pointing to zero degrees but to a division on the outer circle which is to the right or to the left of the zero division depending whether the variation is easterly or westerly. The explanation of this is that the outer divisions in degrees refer to the true North, and are known as true courses, while the inner divisions refer to the magnetic North. It is almost invariably true that when courses are given in degrees they are true courses, and that when they are given in points they are magnetic courses. The magnetic ones are far the more simpler for our use, as we need to take no account of variation whatsoever in dealing with magnetic courses. Variation comes in only when we refer to true courses.

Deviation

There is one error entering into our compasses both when we talk about magnetic, as well as when referring to true courses, and this error is caused by magnetic substances such as iron and steel on our boats. The error caused by the effect which this magnetic substance has on our compass, moving the needle one way or the other, is called deviation. It exists to a greater or less degree on every vessel. Moreover, deviation on any boat is not constant; that is, it is different in amount for every different heading of a boat.

Deviation Varies With Heading

The figure, page 127, shows why this difference in the amount of deviation occurs. Here we have three boats. In the first case, the boat is heading approximately North. The engine diagram is used to represent the center of magnetic attraction on the boat. When the boat is heading approximately North, as shown, the pull of this center of magnetic attraction will be exerted most strongly on the South point of the compass, and in the direction which is approximately Southeast.

As the boat swings around to the easterly direction, it is apparent that the attraction is on altogether different points of the compass. Naturally, this will cause the compass to have a deviation decidedly different from that of the first case.

As the boat swings around to a westerly direction, the pull of the magnetic substance on the boat is again different, and causes an entirely different effect on the compass. In other words, the deviation is different on every different heading.

It cannot be assumed that because we have one point westerly deviation when heading North we will have the same amount when heading East or South.

The Deviation Card

The illustration shows one form of deviation card which is very convenient. The inner compass is magnetic, and the outer one represents the compass on your craft. Put your boat over a number of courses whose magnetic direction can be determined from the chart, and note the headings as indicated by the compass. Draw a line in each case on the above card from the point on the inner (magnetic) compass, representing the chart course, to the outer point which is indicated by your compass.

 
At the left is shown a convenient method of determining one's compass deviation by putting the boat over a number of courses whose directions are known or can be determined readily from the chart. For example, that piece of water shown is part of Huntington Bay. An examination of the chart will show a number of fixed aids to navigation, buoys, headlands, etc. For example, we have bell buoy No. 13-A off Lloyd's Point, a red and black spar off Eaton's Neck, can buoy No. 13 off Eaton's Point, Eaton's Point Lighthouse, Lloyd's Harbor Lighthouse, a light buoy off the entrance to Northport Bay, a white cupola at Eaton's Neck shore line, etc. The magnetic directions between each two of these can be readily determined from the chart itself. It will be found, for example, that the red and black spar bears east by north from the bell buoy No. 13-A, therefore, if one's boat is sailed over this course and the compass indicates any other course than east by north then one will know that he has a certain amount of deviation to this compass on this heading. After this has been observed and by reversing the direction of his boat so that he is running from the red and black spar toward bell buoy No. 13-A, which is fti a west by south direction he will be able to determine deviation of his compass on this heading by observing the direction indicated by his compass and comparing this with the magnetic direction, which is west by south. In the same way when sailing from bell buoy 1 3-A toward the can buoy No. 13, which is in a westerly direction, he can determine his deviation on this heading. In a similar way, following over all possible courses he will be able to get his deviation on a large number of headings

Going over eight courses and the reverse of them will give you the deviation of your compass on sixteen different headings. You will then have a deviation card which will show the compass course which should be steered for any magnetic course.

Easterly And Westerly Deviation

When the north pole of your compass is swung to the right, or toward the East by the magnetic substance on the boat, the deviation is said to be easterly. When the north pole is swung to the left or to the West, we have a westerly deviation. Deviation refers to the north point of the compass, and to no other point—which fact should be remembered by everyone.

Correcting And Uncorrecting Courses

When changing from magnetic to true, by applying variation; from compass to magnetic, by applying deviation; or from compass to true by applying variation and deviation, consider that you are "correcting" courses.

You are "uncorrecting" when changing true to magnetic, magnetic to compass or true to compass.

Applying Deviation

The process of applying deviation to determine compass courses is one which the navigator must do for himself, and make himself a thorough master of. No course can be set or bearing plotted without the application of this problem, and a mistake in its solution may produce serious consequences. Rules as to the application of deviation are of little service. The seaman must practice and work them out for himself. (See also pages 130 and 131 for recommended rules and methods.)

Compass courses and magnetic courses should not be confused. The former is that shown by the compass on your boat, and the magnetic or correct course is the one shown by the chart. To find a compass course when the deviation of your compass is westerly, the compass course which you should steer will be to the right of the magnetic or correct course. In other words, apply a westerly error to the right to find the compass course which should be steered. When the error of your compass is easterly, the compass course which should be steered to allow for this easterly error, is to the left.

To find the magnetic or true course from your compass, just the reverse of the above must be done; apply an easterly error to the right, and a westerly error to the left.

The navigator should always regard himself as facing the point under consideration from a position at the center of the compass when he applies an error.

To Find Compass Course

From the above it will be noted that when the error (variation, deviation, local attraction, or a combination of these) of your compass is westerly, then the compass course which you should steer will be to the right of the magnetic course (course on which it is desired to put the boat, found from the chart or sailing directions) or in other words apply a westerly error to the right to find the compass course which should be steered.

When the error of your compass (variation, deviation, local attraction, or a combination of these) is easterly, then the compass course to be steered by your compass to allow for this easterly error will be to the left of the correct course which you wish to follow.

To Find Correct Course

The correct course or the direction which you really are going is to the right of the course shown by your compass, when same has an easterly error, and to the left of the course shown by your compass when its error is westerly.

When the compass card is arranged from 0° at North, around the card clockwise through the entire 360 degrees, the case becomes much simpler and is easy to memorize. In this case, when it is desired to change a true course into a magnetic course or compass course, or to change a magnetic to a compass course, the variation or deviation, as the case may be, should be added when same is westerly and when the error is easterly its amount should be subtracted.

When it is desired to change a compass course into either a magnetic or true course, add an easterly error and substract a westerly error from the compass course, or when changing a magnetic course to a true course add an easterly variation and subtract a westerly deviation from the magnetic course given.

When the compass card is arranged in points instead of in degrees, to get the magnetic or compass course from the given true course when the variation and deviation are also given, imagine your own position at the center of the compass card and apply an easterly error to the left and a westerly error to the right; the term error meaning the algebraic sum of the amount of deviation and variation, if the case requires that both be used, or if not, then to the amount of either variation or deviation which must be taken into consideration.

To find correct course or the direction which the vessel is really heading when there is a compass error, apply the amount of the error to the right, if the error is east, and to the left if same is westerly.

Examples

Example 1:                                           Example 2:

True Course: SW                                 Compass Course: SE
Variation: 1 point W                             Deviation: 2 point E
Find: Magnetic Course                          Find: Magnetic Course
Answer: SW x W                                 Answer: SE ½ S

Example 3:                                           Example 4:

True Course: SW                                 Compass Course: SE
Variation: 1 point W                             Deviation: 2 point E
Deviation: 2 point E                              Variation: 1 point West
Find: Compass Course                         Find: True Course
Answer: SW 2 W                                 Answer: SE 2 E

Example 5:                                           Example 6:

Magnetic Course: NW                          Magnetic Course: NE
Deviation: 1 point W                             Variation: 2. point W
Find: Compass Course                         Find: True Course
Answer: NW x N                                 Answer: NE 2 N

Correct Easterly Errors Clockwise (CEEC)

If the sailor will simply memorize the expression "Correct Easterly Errors Clockwise" or its symbol "C E E C", every problem in deviation, variation or both can be readily solved without the necessity of memorizing further formulae.

The diagram on page 132 indicates the proper solution of all problems involving true, magnetic or compass courses or any combination of these in the application of variation or deviation or both.

Simply remember the expression "C E E C," "Correct Easterly Errors Clockwise" and that when you change any factor of "C E E C" (as for example when you have a westerly error or when you are "uncorrecting" instead of "correcting"), you must change two of them but only two of them.
There can be but a total of three correcting problems, namely:

(1) Changing magnetic to true (by applying variation).
(2) Changing compass to magnetic (by applying deviation).
(3) Changing compass to true (by applying variation and deviation).

In all of these three cases (when the error is easterly) apply CEEC (Correct Easterly Errors Clockwise) or in the case of westerly errors (by changing two and only two of the factors in CEEC) apply C W E CC (correct westerly errors counter-clockwise).

The same rule holds equally good in the three un-correcting problems, as is illustrated in the diagram on page 132.

By changing two of the factors and only two in "CEEC" every problem in variation and deviation may be solved.

Post such a diagram alongside the compass on your boat.

CORRECT EASTERLY ERRORS CLOCKWISE

If the Expression ``CEEC" is Kept in Mind, All Problems of Compass Errors and Their Application May Be Solved

CORRECTING (C)

is changing (1) Magnetic to True by applying Variation
or changing (2) Compass to Magnetic by applying Deviation
or changing (3) Compass to True by applying Variation and Deviation

UNCORRECTING (UC)

is changing (4) True to Magnetic by applying Variation
or changing (5) Magnetic to Compass by applying Deviationor
changing (6) True to Compass by applying Variation and Deviation

WHEN CHANGING CEEC

for westerly (W) errors, and for uncorrecting (UC):-
Always change two of the factors (arid only two) in the basic CEEC formula. This will give the following combinations which will solve all problems:-

C Correct         C Correct         UC Uncorrect              UC Uncorrect
E Easterly         W Westerly      E Easterly                     W Westerly
E Errors           E Errors           E Errors                       E Errors
C Clockwise    CC Counter     CC Counter                 C Clockwise
                        Clockwise        Clockwise                   

Note: Do not try to memorize all of the· combinations. Simply remember "CEEC" and that to obtain •all other combinations you must •always change. two of the· factors in CEEC and only two.

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