Compass errors

by Don Launer

Getting a magnetic compass to tell the truth about North

Most recreational sailors today navigate their craft using electronic devices. This is all well and good until an electrical failure on board shuts them down or when the GPS fails (see “GPS Vulnerabilities 101,” November 2010). In either of these events, they need an alternative navigation system. Loran is no longer operational, so sailors must go back to the navigation techniques used before electronic navigation systems existed — navigating by compass. Most boats have a non-electronic magnetic compass on board, but how many skippers know how to navigate with a magnetic compass? And do they know whether or not their boats’ compasses are reliable?

A compass uses Earth’s magnetic field to provide directional information, but aboard a boat there are other magnetic fields to lead it astray. Moreover, Earth’s magnetic field does not align conveniently with the lines of longitude we use for navigating.

The wandering poles

Earth’s magnetic poles have been wandering around the north- and south-polar regions for millions of years independent of each other. Data collected from the 1500s until the present show that, during this relatively brief period in the history of the Earth, the magnetic north pole has made a trip from the Arctic Ocean into northern Canada north of Hudson Bay and back to the Arctic Ocean at an average speed of about 10 to 15 miles per year. It’s impossible to predict where the magnetic poles will go next. Larry Newitt of the Geological Survey of Canada says, “Although it has been moving north or northwest for a hundred years, it is not going to continue in that direction forever. Its speed has increased considerably during the past 25 years and it could just as easily decrease a few years from now.”

In addition to the long-term movement of the magnetic poles there is also a daily (diurnal) movement of the poles. This daily movement follows a roughly elliptical path around the pole’s average position. The path is sometimes a very small ellipse and sometimes a very large one — more than 100 miles during a 24-hour period. It’s believed that this diurnal movement is caused by the solar wind of charged particles streaming from the sun, and that solar storms on the surface of the sun can influence the size of this elliptical path.

Variation and isogonic lines

Lines of longitude on a chart are aligned north and south, terminating at the Earth’s axis at 90 degrees north latitude (the North Pole) and 90 degrees south latitude (the South Pole). Magnetic north is not at the North Pole, and the angular difference between the direction toward the North Pole — true north — and the direction toward the magnetic north pole — magnetic north — is called variation. Variation is expressed in degrees east or west of true north.

Variation is different from place to place on Earth’s surface and, since the magnetic poles are continuously migrating, it also changes over time. The variation in London, for example, changed more than 34 degrees between 1580 and 1850 due to the movement of the magnetic north pole. Unfortunately, mariners in those days often ignored variation (or were ignorant of it) and ships and lives were lost as a result.

The magnetic lines of force around Earth are extremely irregular, primarily due to the non-uniform distribution of ferrous material inside Earth. Lines that connect points where variation is the same are known as isogonic lines.

In 1700, astronomer and mathematician Edmond Halley (1656-1742) — for whom the comet and space telescope are named — made one of many scientific voyages to measure variation. He returned with a huge amount of data that were used to create what he described as “Magnetic Curve Lines” connecting points of equal variation. This was the first isogonic chart.

While isogonic charts show the distribution of variation worldwide, they don’t show the many localized irregularities that, in many cases, can be enormous. Off the Australian coast, for example, there is a position where, in the distance of two football fields, the variation changes by 90 degrees!

Many local variations (called magnetic anomalies) are the result of volcanic eruptions that have deposited iron-rich lava on the ocean floor. Changes in compass readings caused by these iron deposits were first recorded by Icelandic sailors in the 1700s.

An isogonic chart displays lines of equal magnetic variation. The outer circle of a compass rose is marked with bearings relative to true north. The inner circle is offset from it by the magnetic variation at that location.

The compass rose

It was Robert Norman, of England, who first distilled variation down to a simple diagram, the compass rose, which is still found on all paper charts today.

The nautical chart’s compass rose presents information about variation in a convenient, simple, and usable form. The outer ring of the rose shows bearings to true north or south, which coincide with the lines of longitude. The next inner ring of the rose shows magnetic bearings — directions relative to the magnetic north and south poles. In the center, the current variation for the region covered by the chart is printed in degrees east or west together with the date of this variation and the annual predicted change. Since the movement of the magnetic poles is erratic and unpredictable in the long term, this predicted annual change in variation is only applicable for a few years from the date shown on the chart. Nevertheless, the yearly change can be extrapolated with reasonable accuracy on charts that are just a few years old.


A current flowing through a wire creates a magnetic field around that wire, and the greater the current, the stronger the magnetic field. Most of this magnetism is cancelled out if wires are run in pairs, that is, run close together or twisted together, where one wire carries current to the load and the other carries the return current from the load.

Electrical appliances or navigation instruments can also be sources of unwanted magnetic fields. In particular, loudspeakers that contain strong permanent magnets produce very strong magnetic fields and should never be installed or placed anywhere near the compass.

Alternators, when operating, can also produce a substantial magnetic field. If your compass is within several feet of your engine’s alternator, you will probably see that the compass heading changes when the engine is running and driving the alternator.

Large masses containing iron, such as the engine, affect magnetic fields around them (including Earth’s) and can even be magnetized themselves.

With all these stray magnetic fields aboard a boat, most magnetic compasses don’t actually point to magnetic north at all. The difference between the true magnetic heading and the heading that the compass is showing is termed deviation, and deviation is particular to every individual boat. In fact the deviation aboard a boat can change due to simple things like canned food being stowed too close to the compass.

Deviation was not a big problem aboard wooden sailing ships, since the amount of ferrous metal on board was minimal. However, in 1627, Captain John Smith (1580-1631) suggested that wooden pegs be used as fastenings when a binnacle was constructed to house a compass because the iron nails commonly used could throw the compass heading off.

When iron ships came on the scene, deviation was of real consequence until a method of compensating for the error with the use of soft iron and magnets was proven. Compasses used on sailboats today have small magnets built into their housings. Positioned 90 degrees apart, they can be rotated using a non-magnetic screwdriver through screw slots in the case.

Conquering deviation, though, is not simple, since deviation varies with the boat’s heading or angle of heel. Owners of steel boats have to consider another factor: if a steel boat continues on the same course for a long time while being battered by seas, it can become re-magnetized by the Earth’s magnetic field, perhaps in a new direction, and the deviation cards will no longer be accurate.

Deviation card

Correcting the deviation completely is a complicated process and tends to involve a combination of art and science. Thankfully, manufacturers of compasses for recreational boats have given us only two adjustments: the two small bar magnets inside the compass case.

The procedure during which these two magnets are adjusted is known as swinging the compass, or swinging ship, and requires the boat to be steered on the cardinal headings (N, E, S, and W). It is usually outlined in the instruction sheet that comes with the compass and can also be found in Chapman Piloting and Seamanship and other books. For those who consider the process daunting, there’s always the option of hiring a compass adjuster. It will be money well spent. The adjuster will adjust the compass to take out as much of the deviation as possible and also make up the deviation card.

Swinging the compass is a good project for a calm day when you’re waiting for the wind to pick up. The effort may repay you at some unexpected time in the future.

The conventional way to make a deviation card is to motor the boat along known bearings (set up by ranges on fixed marks ashore or GPS bearings toward landmarks at known positions) and comparing the compass heading to the known or GPS magnetic bearing. Doing this every 15 or 20 degrees around the compass gives enough readings.

To make the deviation card for your boat, plot these readings as points on a graph and draw a smooth line through them. Some of the points might not lie exactly on the curve but you will find that the curve is very close to a sine wave and that the deviation error is zero on two headings only.

You might want to take the boat out again and check the accuracy of the card with the engine running and when navigation lights and other electronic gear are turned on.

Once you have made and double-checked your deviation card, laminate it and keep it near the compass or in a handy spot down below in preparation for that time when your compass will be your primary navigational tool. It’s possible that, if your compass is close to the engine’s alternator, you might need two deviation charts, one for when you’re sailing and another for when the engine is running.

Heeling error

There is one additional concern: heeling error. This error can be introduced when your boat is heeled over and the boat’s ferrous metals and magnetic fields change position in relation to the gimbaled compass. Since this might change the deviation error, it’s a good idea to double-check your deviation card when you’re sailing to windward at a good angle of heel.

Iron ships and deviation

In the early 1800s, the first ships built of iron began to sail the oceans. Huge compass errors resulted from deviation and led to many disasters.

Ferrous metals, such as iron, and to a lesser extent steel, have a unique quality. If they are pounded in the presence of a magnetic field, they take on the direction of that field and become magnets themselves. Iron ships were fastened with rivets, which are headed metal pins or bolts, usually iron or steel, used for uniting metal plates. A rivet is heated red hot and, with the head on the outside, the shank is passed through the holes in the plates. The end of the shank is then hammered down to form another head on the inside. When the rivet cools and contracts, it joins the plates tightly together.

Tens of thousands of rivets were used in the construction of a ship. If the ship was being constructed with its keel aligned in a north-south direction, the hammering of thousands of rivets would cause the ship to take on Earth’s magnetic field and, in essence, become a huge magnet . . . so much so that a compass at the bow would often read 180 degrees away from one at the stern. The magnetism in the hulls of those first iron ships caused so much difficulty it was said they could never be successfully navigated and were unsafe.

The principle of deviation was not well understood in the early days of iron vessels, but distancing the compass from the iron hull was found to reduce its effect. One way this was achieved was to mount the compass atop a pole, often so high that a ladder was required to read it. Another method for getting a low-deviation reading was to take a compass up to the crow’s nest.

In 1838, the head of the Royal Navy’s hydrographic office, Francis Beaufort — who originated the Beaufort Wind Scale — asked scientist and Astronomer Royal, George Airy, to examine the deviation problem aboard iron ships and suggest a solution. Airy was able to reduce the errors to nearly zero by placing several bar magnets and pieces of soft iron in strategic positions near the ship’s compass.

Later, when deviation was more fully understood, shipwrights constructed brass binnacles and fitted a large iron ball on each side of the compass and close to it. The athwartships positions of these balls relative to the compass could be adjusted to take out a large amount of the deviation, but magnets and iron bars were still needed inside the binnacle for further compensation.

To this day, recreational boat compasses have tiny magnets inside their cases that can be adjusted to reduce the effects of deviation, just as Airy taught us in the early 1800s.

Don Launer, a Good Old Boat contributing editor, built his two-masted schooner, Delphinus, from a bare hull. He has held a USCG captain’s license for more than 38 years and has written five books.

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