Understanding and Using the Magnetic Compass - SailNet Community
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Understanding and Using the Magnetic Compass

An ocean navigator must have a fundamental background in navigation to ensure an accurate positioning of the vessel. Dead reckoning procedures, aided by basic instruments, will give you the foundation that can help solve the three basic problems of navigation: position, direction to destination, and time of arrival. It is possible, using only a compass and knot meter, to navigate directly to any place in the world. The compass is perhaps the most important instrument on board your vessel. Understanding its operating principles, and correctly using it for navigation, is the hallmark of a successful navigator.

Present day magnetic compasses use the same forces that guided ancient mariners. A magnetized needle, in conjunction with a compass card, rotates horizontally. Present day compasses are superior to the ancient ones through a heightened knowledge of magnetic laws and greater precision in construction.

The Earth's magnetic lines of force provide the directional information needed to navigate. A compass detects and converts the energy from these magnetic lines of force into a directional display. In order to understand the operation of a ship's compass, it is first necessary to understand some basic information about the Earth's magnetic field.

The Earth has some of the magnetic properties of a bar magnet; however, its magnetic poles are not located at its geographic poles, nor are the two poles located exactly opposite each other as on a straight bar magnet. The Earth's magnetic poles can be considered to be connected by a number of lines of force emanating from the south magnetic pole and terminating at the north magnetic pole. These irregular, curved lines are called magnetic meridians. The angle formed at any point between a magnetic meridian and the geographic median (a straight line between the geographic north and south poles) is called variation. Lines connecting points having the same magnetic variation are called isogonic lines.

The local variation and its small annual change are noted on the compass rose of all navigational charts. Variation is listed on the chart as east or west. When variation is east, magnetic north is east of true north. Similarly, when variation is west, magnetic north is west of true north. Correction for magnetic variation must be calculated if a compass direction is to be converted to a true direction, or vice versa.

The force of the Earth's magnetic field can be divided into two components: the vertical and the horizontal. The relative intensity of these two components varies over the Earth. At the magnetic poles, the vertical component is at maximum strength while the horizontal component is at its minimum strength. At approximately the midpoint between the two poles, the horizontal component is at maximum strength and the vertical is minimal. The magnetic compass indicates direction in the horizontal plane with reference to the horizontal component of the Earth's magnetic field. Therefore, a compass loses its usefulness in areas of weak horizontal forces such as the area around the magnetic poles.

Secondary magnetic fields in the vicinity of the compass can also affect the compass readings. These secondary magnetic fields are caused by the presence of ferromagnetic objects, electronics, and electrical wires in the boat. This error can be reduced by changing the position of the small compensating magnets in the compass case. However, it is not possible to remove all of these errors on all headings. The end result will be a compass card showing the number of degrees of error in the compass when you are on various compass headings. This error is called deviation, and like variation must also be considered when determining heading relationships.

The correction for variation and deviation is usually expressed as east or west and is computed as a correction to true heading. In order to make this computation easier we usually convert the east or west values to a plus or minus value and add them algebraically. If variation or deviation is east, the sign of the correction is minus and if west, the sign is plus. A good mnemonic for remembering this is "east is least and west is best."

Ship's headings are expressed in various ways, according to the basic reference. If the heading is measured in relation to geographic north, it is a true heading. If the heading is in reference to magnetic north, it is a magnetic heading and if it is in reference to the compass lubber line, it is a compass heading. The reason we need to know all of this is that directions on a chart are in relation to true north, while we must use a magnetic compass to steer. Therefore, if you draw a line on the chart from point A to point B and measure the true course between these points, you must then convert this true direction to a compass heading so you can steer the boat from point A to point B. The other side of this coin is that all bearings and courses taken from the ship's compass will be compass bearings or compass courses and must be converted to true bearings or true courses in order to plot them on the chart. While it is true that you can use the magnetic portion of the compass rose to measure and plot magnetic directions, the preferred and professional method is to use always true courses and bearings on the chart and mathematically convert them either to or from compass courses and bearings.

Let me summarize these heading relationships: Deviation is the difference between the compass heading and the magnetic heading. Variation is the difference between the magnetic heading and the true heading. The algebraic sum of deviation and variation is the compass error.

In order to go from compass to true, use the mnemonic "Can Dead Men Vote Twice At Elections" to remember the conversion process (Compass, Deviation, Magnetic, Variation, True, Add East.) When converting compass heading to true heading, add east deviation and variation and subtract west deviation and variation.

To convert from true to compass, use the mnemonic, "T. V. Makes Dull Children All Ways" (True, Variation, Magnetic, Deviation, Compass, Add West.) When converting true heading to compass heading, add west deviation and variation and subtract east deviation and variation.

Here are a few examples of the conversion process:

 Compass Deviation Magnetic Variation True 358 5E 003 6E 009 120 1W 119 3E 122 180 6E 186 8W 178 240 5W 235 7W 228

 True Variation Magnetic Deviation Compass 009 6E 003 5E 358 122 3E 119 1W 120 178 8W 186 6E 180 228 7W 235 5W 240

I know that a GPS unit not only will display true course, but the ones that calculate variation can also display magnetic heading. I hear all the time from beginners that they don't need anything but their GPS. However, the importance of being able to understand and use your compass is undeniable, since it is totally independent of your electronics and ship's power. When all else fails, the compass will still be there to help get you home safely.... if you know how to use it.

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Jim Sexton is offline

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