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"Teaching people about the world in which we live."

Compasses
by Shawn Steele

A compass can tell you which direction you are going, this is known as your heading. A compass can also tell you the direction an object is from you. Those directions are known as bearings. Once bearings of objects are known, you can use a map or a chart to find out where you are. You can then figure out which direction you want to go and use the compass to make sure you follow that heading.

A compass works by using a magnetized needle that orients itself with the earth's natural magnetic field to point in the direction of magnetic north. The bearing or heading is then read from numbers on the compass to indicate directions such as north, south, east, or west.

The numbers that are on a compass represent degrees of a circle. The degrees are numbered from 0° to 360°, clockwise around the circle, with 0° and 360° being the same thing. 0° (and by default 360°) represents north; 90° represents east; 180° represents south; 270° represents west.

If your heading, the direction you are going, is toward the east, you are said to be on a heading of 90°. Likewise if your heading is toward the west, you are said to be on a heading of 270°.

If you are on a boat (Boat A)and there is a lighthouse directly east of you, the bearing of that lighthouse is 90°. Another lighthouse directly south of you, would be at a bearing of 180°. It does not matter which way your boat is pointing, what we are measuring is the direction of the lighthouse from the boat as it would be shown on a chart.

Reading a Compass

Compasses are sensitive to magnetic fields, and can be affected by small metal or magnetic objects. If a compass is used near a metal belt buckle, a camera, a radio, or other metal or electronic items, it may not be accurate. The first step when using a compass is to make sure that there are no metal objects nearby. For some compasses, such as those on a metal ship or airplane, this is not possible. In those cases special steps are taken to correct for the "compass error", also known as magnetic deviation. Note that magnetic deviation is different than magnetic variation, both of which are described below.

Compasses can either be hand held or mounted on an object such as a boat. A hand held is used by pointing it toward the object you are interested in. The marks on a compass are then read to find the bearing of the object.

A compass attached to a boat should be installed so that the bearing taken is of the heading of the boat is in the direction indicated by the compass. Taking a bearing of an object not directly ahead of the boat can be difficult, and in those cases a hand bearing compass might be preferred.

Bearings taken from a compass are usually read in degrees of a circle with 0° being toward the magnetic north pole, 180° is south, 90° is East and 270° is West. Bearings taken by a compass are magnetic bearings, and not true bearings. When using a compass, you need to take into account the difference between true and magnetic north.

True North and Magnetic North

True north is the direction toward the geographic north pole. The earth rotates around a line between the geographic north and south poles. Most globes can spin on that line, just as the earth does in real life.

Magnetic north is slightly different. Magnetic north is the direction toward the magnetic north pole. The earth's natural magnetic field passes through the north and south magnetic poles.

Magnetic Variation

Magnetic variation is the difference between true north and magnetic north. This is important because a compass points toward magnetic north, but maps and charts are made with respect to true north. Magnetic variation can vary tremendously from place to place, and can even be affected by such things as mineral deposits in the ground.

Compass Rose

Charts and maps should have the magnetic variation printed on them, either near the legend or as part of the compass rose. Magnetic variation will be labeled in degrees east or west, such as "variation 6°15'W (1985) annual decrease 8'." If the chart has more than one compass rose with magnetic variation labeled, the one closest to where you are should be used. Some charts may also have lines joining points of equal magnetic variation, known as isogonic lines.

Notice that the example said "annual decrease 8' (minutes)." The earth's magnetic field moves, so it is important to know what the current magnetic variation is. Assuming it is now 1997, the variation has changed by 96 minutes (8 minutes per year times 12 years.) A minute is 1/60th of a degree, so that is about one and a half degrees of change. In our example the variation is now 4°39'W.

Now that we know the current variation for our area, we can figure out what our true bearing or heading is from our compass reading. Variation measured in degrees west is subtracted from the compass bearing to get our true bearing. Variation in degrees east is added to the compass bearing to get the true bearing.

Let's assume our compass bearing was 45°, what is our true bearing for this example? 45° - 4°39' = 40°21'. Remember that there are only 60 minutes in a degree, 60-21=39.

It works the other way around too. To find magnetic courses from true courses, variation east is subtracted and variation west is added. If we want to sail a true course of 90°, what would we have to read on the compass? 90° + 4°39' = 94°39'. We would probably round that to 95° since it would be hard to see the 39' on a compass.

Usually magnetic variation is corrected for by using a table such as the examples at the bottom of this page. On such a table, the true course would be labeled "T," the magnetic variation "V," and the magnetic course "M."

Magnetic Deviation or Compass Error

When a compass is located near a metallic, magnetic, or electrical object, it may not point in the correct direction. The difference between the direction of magnetic north and the direction that the compass points is known as magnetic deviation. A compass located on an object such as a boat or airplane will probably have some magnetic deviation.

We use a compass to tell us where magnetic north, and indirectly true north, are. That makes it important to know how much deviation or error there is in our compass. The process of checking for magnetic deviation is known as "swinging" the compass and is fairly straightforward. To swing a compass, the compass is pointed in a direction known to be magnetic north. The difference between the compass reading and what it should read is then recorded, in degrees east or west.

Correcting for magnetic deviation is similar to correcting for magnetic variation. Once you have a magnetic course, you add or subtract the appropriate error to get a compass course. When going from magnetic course to a compass course, errors labeled west are added and east are subtracted. Usually these calculations are done on a chart with the magnetic course labeled "M," the deviation labeled "D," and the compass course labeled "C."

Correcting for Magnetic Variation and Deviation

Here are some examples of how these errors are corrected for:

Example 1:

When going from true course to compass course, which is down this table, variations and deviations labeled WEST (W) are ADDED. Likewise variations and deviations labeled EAST (E) are SUBTRACTED.

Step 1: Our variation is west, so to find our magnetic course we must add. 45°15' plus 12°10' equals 57°25'.

Step 2: Similarly, our deviation is east, so we subtract to find the compass course. 57°25' minus 7°15' equals 50°10'.

If we had been going up the table instead, from compass course to true course, we would do the opposite. For easterly variations and deviations we would add, and for westerly ones we would subtract.

Example 2

This time the variation is east, so we will be subtracting as we go from true to magnetic courses. The catch is that there are only 60 minutes in a degree, so as we subtract 39 from 5 we get -34 minutes. To fix this we must add 60 minutes, and subtract 1 additional degree. So 5-34+60=26 minutes and 206-4-1=201 degrees.

The deviation is west, so we will add as we continue from magnetic to compass courses. Again, there are only 60 minutes in a degree and as we add 45 to 26 minutes, we get 71 minutes. To correct this, we have to subtract 60 minutes and add 1 extra degree. As we see, 45+26-60=11 minutes and 201+3+1=205 degrees.

Example 3

In example 3, both the variation and the deviation are west, so we will add both times as we go from true to compass courses. When we add the variation, we notice that again the minutes is over 60, so when we carry the extra degree we end up with 360 degrees, which is the same as 0 degrees. We also have to carry when adding the deviation, so we end up with at total of 9 degrees, 7 minutes.

Things to Remember

• To remember the order of the items on the table TVMDC, remember "TV Makes Dull Company." T=True Course, V=Variation, M=Magnetic Course, D=Deviation, C=Compass Course.
• To remember whether to add or subtract try "Variation West, Compass Best. Variation East, Compass Least." That reminds you that if the variation is west, the compass will be HIGHER than the true course, so you must add.
• There are sixty minutes in a degree, so if you get more than 60 minutes or less than 0, you must carry from one of the degrees. If you need to add 60 minutes, subtract 1 degree. If you need to subtract 60 minutes, add 1 degree.
• There are 360 degrees in a circle, so if you end up with more than 360 degrees, just subtract an extra 360 degrees. If you have less than 0 degrees, add an extra 360 degrees to your result.

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