The Errors of the Magnetic Compass and Their Correction

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Why is a magnetic compass required in the age of electronic navigation?

Despite the modern tendency to rely heavily on Electronic Navigational Aids (ENA) , the magnetic compass remains an essential navigation instrument on any sea going vessel, and continues to operate independently, in the not uncommon event of an electrical failure or electronics malfunction. Users should be aware that ENA have limitations and have been known to provide erroneous information. Reliable and accessible alternatives for back up and cross reference should always be readily available . Vessels are required to be equipped with a means of determining direction and heading, readable from the steering position and independent of any power supply . A correctly installed and adjusted magnetic compass, of a size and type suitable for the vessel, fulfills this requirement.

Other uses for a magnetic compass

There is little doubt that Global Navigation Satellite Systems (GNSS) , such as GPS , help to make modern sea travel generally safer, and for navigators, in many respects, easier than it used to be, particularly when interfaced with A.I.S. , radar and electronic chart display systems such as ECDIS . It is, however, worth taking the following into consideration:

GPS is currently the only fully operational GNSS. It is owned and controlled by the U.S. Department of Defence and its commercial and recreational use is incidental to its primary, military purpose.

GNSS signals are vulnerable to loss and error, both intentional and unintentional. Malicious jamming of GNSS is a very real threat. GPS signals can be terminated or corrupted by the US military for security purposes.

Commercial GPS operates on a single frequency only. Military GPS receivers operate on a dual frequency system which is more reliable and less vulnerable to error caused by atmospheric conditions.

GNSS signals are extremely vulnerable to solar activity such as solar flares . The sun is currently entering a phase of intense solar flare activity which is due to last for several years.

Some areas of the world, particularly in the higher latitudes, have problematic or no GNSS/GPS coverage.

Other signal errors, such as multipath effect , occur locally when the signal to the antenna is reflected off nearby objects, such as superstructure, masts and funnel . Entering the wrong antenna height into the receiver can cause significant errors. E.G. the difference between the antenna of a large vessel in ballast and sea level. Entering the wrong datum can put the vessel's position miles from where it really is. Default datum used in GPS calculations is WGS84 . In some areas of the world electronic chart coverage is by raster charts (scanned paper charts) alone. The datum of many raster charts is not WGS84. When GPS shows a compass course, it is not showing the ship's heading, it is showing the track of the vessel - where she has been in relation to her current position. With the vessel stationary, GPS will not provide any directional information.

Failure to observe ENA errors was a major factor in the grounding of this U.S. warship

Most electronic compasses (GPS and gyro compasses are two exceptions) are effected by magnetic deviation. They are also reliant on a power supply. Electronic compasses used for marine navigation, include:

GPS Compass - comprising 2, or preferably 3, antennas aligned symetrically fore and aft, will show the ship's heading, in either true or magnetic form, and is normally accurate to within +/- one degree on a steady heading . As with all satellite derived data, it is vulnerable to signal error and reliant on a supply of electricity.

Fluxgate Compass - uses a number of electrical coils wound on a magnetic core to detect its alignment with the magnetic meridian. It will also detect any other magnetic fields around it and is therefore as susceptible to deviation as the standard compass.

Electro-Magnetic Resistors - used in some electronic compasses to measure the earth's magnetic field. As the vessel changes direction or alignment with the magnetic meridian, resistance increases or decreases and is interpreted as heading.

Gyro Compass - usually fitted on larger vessels. It is set to point true north and does not use the earth's magnetic field. It is nomally accurate to +/- one or two degrees. Modern fibre optic gyro compasses are continuously corrected by computers, which are updated from GPS. It can take many hours for a gyro compass to operate correctly from the time it is switched on, or switched back on , after a power outage.

Laser and Atomic Compasses - still in early days of development for commercial marine use but may be commonplace in the not too distant future.

In Summary - State of the Art Technology can be a great asset to the modern seafarer - when it works properly. As we all know, it sometimes doesn't, and then things can very quickly turn pear shaped. User error due to inadequate training, fatigue and "information overload" can also contribute to innaccuracies and misinterpretation of data. Over reliance on electronic navigation aids leads to complacency and sometimes to disaster.

In recent years, there have been numerous well documented occasions (and many not so well documented) on which a sudden, unexpected loss of power or the undetected inaccuracy of electronic instruments, has rapidly developed into a serious crisis. Very often, the ability and readiness to switch to old fashioned "manual" navigation, including the use of a reliable magnetic compass (and looking out of the window!), has made the difference between continuing the voyage safely and a major marine incident.

Over reliance on electronic navigation aids can lead to trouble

Magnetic compass location and installation

On most large merchant vessels the standard compass is installed on the "Monkey Island", i.e. above the wheelhouse. It is usually viewed from the helm via a viewing tube, similar to a periscope. Often, electronic repeaters are installed so that compass headings can be viewed around the wheelhouse. Being installed on the highest deck of the ship enables it to be used for taking bearings and keeps it as far away from magnetic interference as possible.

Smaller merchant vessels and warships often have a steering compass installed inside the wheel house in front of the helm. In a fully enclosed steel wheelhouse a magnetic compass is bound to be affected by a number of deviating magnetic fields and a certain amount of skill is required on the part of the compass adjuster to compensate for these.

Compass installation on the Monkey Island on vessel's centre line

Ideally, the compass should be installed on the vessel's centre line so that deviating magnetic forces are mostly symetrical around the compass. On certain vessels, such as aircraft carriers, some fishing vessels and some modern container ships with a narrow superstructure section, the compass is offset, and this can create interesting challenges for compass adjusters.

On small vessels the compass is usually located in front of the helm position. Care should be taken to ensure the compass is installed far enough away from structural members, equipment and instruments such as radios, speakers, engine rev counters (tachometers), etc, which can produce strong magnetic fields. A few inches one way or the other can sometimes be the difference between major and minor deviation.

Magnetic compass & electronic instruments in close proximity

It should be installed so it is easily readable from the helm and also accessible for adjusting. A great many modern vessels, particularly luxury motor yachts, have not been designed with this in mind. On one particular sleek, multi-million dollar super yacht, it was found that, in order to access the integral correctors of the flush fitting compass, either the console would need to be partially demolished or the raked wheelhouse windscreen would have to be removed. Ideally, the compass should be sited so that bearings of objects and other vessels may be taken. This is not always practicable, particularly on smaller vessels, in which case other means of taking bearings should be provided. It should not be forgotten that the compass is a valuable tool in collision avoidance.

Overhead mounted compass

Some vessels have their compass installed in an overhead, deckhead mounted position. A number of manufacturers produce compasses which can be mounted in this fashion. This has an obvious advantage in being easy to read close to eye level. In an "upside down" type, such as that pictured above, it also means that air bubbles in the compass liquid are not such a problem. It is also away from a lot of the deviating magnetic fields often found around a console mounted compass.

Suffice to say, all fastenings used to install the compass should be of non-ferrous, non-magnetic material, e.g. bronze or marine grade stainless steel.

Variation, deviation and compass correction

MAGNETIC VARIATION ( or DECLINATION ) is the difference between True North and Magnetic North . It is due to t he earth's magnetic field, which travels from South to North, not travelling in a straight line. In some locations, variation can be in excess of 30 degrees. In some locations it is zero.

It is also interesting to note that the Magnetic North and South Poles are located considerable distances from the Geographic North and South Poles respectively. (The Magnetic North Pole is, at time of writing, over 1,000 miles from the Geographic North Pole and this distance is currently increasing by about 40 miles a year).

The map below shows the world's magnetic variation values. Red lines indicate easterly variation and blue lines indicate westerly variation. The green lines indicates zero variation.

To find magnetic variation and other magnetic values for a particular location, click on map .

The compass is said to be pointing Magnetic North when it is perfectly aligned with the earth's magnetic field - along the magnetic meridian. Therefore, the direction of Magnetic North will vary between zero degrees and in excess of 30 degrees to east or west of true north, depending on the location.

COMPASS DEVIATION is the difference between Magnetic North and the direction in which the compass is pointing. Both variation and deviation are measured in degrees east (+) or west (-).

Easterly deviation should be added to the compass heading to give the magnetic heading and westerly deviation should be subtracted.

Remember: ''ERROR EAST - COMPASS LEAST''

Similarly, easterly variation must be added to the magnetic heading to give the true heading and westerly variation must be subtracted.

CAUSES OF DEVIATION - All vessels have numerous magnetic fields. Some of these fields are permanently built into the structure of the vessel and some are caused by the type of cargo carried, electronic instruments, electrical appliances, position of machinery and equipment, etc. These magnetic fields can combine to cause the compass needle to point away, or deviate , from magnetic north. The amount of deviation can vary considerably from heading to heading as the vessel's magnetism is influenced by the earth's own.

The vessel's soft iron magnetism changes with the orientation and location of the vessel and is also known as

induced magnetism. Hard iron magnetism remains constant, is built into the vessel and is also known as permanent magnetism. Equipment, machinery, cargo, etc can produce both permanant and induced magnetic fields.

The aim of the compass adjuster is to nullify the effect of the unwanted magnetic fields by placing correctors (magnets and soft iron) adjacent to the compass. These create equal but opposing magnetic fields, thus eliminating the deviating fields around the compass, enabling it to align correctly. Each axis, vertical, longitudinal and athwartships is treated seperately.

Some cargoes can affect the compass more than others

Swinging the compass or swinging the ship

Swinging the compass , or swinging the ship (as the operation is sometimes more accurately called as the ship swings around the compass card which, ideally, remains pointing north), involves taking the vessel to a suitable location in open water with plenty of room for manoeuvring. With the vessel steady on each of the eight primary compass points, existing compass headings or bearings are compared with what we know the actual magnetic headings or bearings should be, the difference being the deviation.

During the process, any magnetic fields, created by the ship's structure, equipment, etc, which cause the compass to deviate are reduced or, if possible, eliminated, by creating equal but opposite magnetic fields using compensating correctors. These are placed inside the compass binnacle or adjacent to the compass:

Magnets are aligned fore and aft and athwartships to create horizontal magnetic fields to compensate for the permanent horizontal components of the ship's magnetism.
Soft iron correcting spheres or plates and the Flinders bar compensate for the induced magnetism caused by the effect the earth's magnetic field has on the ship's magnetism.
Heeling error magnets compensate for the vertical component of the ship's magnetism.

The timing and logistics of this operation are often governed by the tide, the weather and other vessels in the vicinity. The time it takes to swing and adjust the compass is also influenced by the condition and accessibility of the compass and correctors, the manoeuvrability of the vessel, the skill of the helmsman and the complexity of, and reasons for, the deviating magnetic fields involved. On successful completion of compass swing, a table recording any remaining residual deviation and a statement as to the good working order of the compass will be issued. A current deviation card / certificate of adjustment is a legal requirement on all sea going commercial vessels.

Using a shadow pin and the sun to check the compass for deviation

Deviation can be determined by a number of methods: the sun's azimuth or known bearings of distant objects, such as a mountain peak or lighthouse are considered most accurate. In certain circumstances, such as poor visibilty, calibration is carried out by making comparisons with other navigation instruments, such as a gyro or GPS compass. Using other navigation instruments to find deviation is only satisfactory if the absolute accuracy of these instruments has first been verified, or any known error is factored into the calculations. Most professionals prefer something tangible, such as a fixed landmark, with a known position and bearing to work with.

GPS compasses are normally accurate to within a degree or so with the vessel on a steady heading but are often far less accurate on a swinging vessel. All navigation instruments, whether portable or fixed, including GPS compasses, should themselves be checked for error each time they are used for calibrating a magnetic compass.

Signal flags "OSCAR" over "QUEBEC" - Denotes swinging the ship

Why can't the compass be swung with the ship alongside?

It should be noted that the compass adjustment cannot be completed to any verifiable accuracy without deviation being observed and adjustments made with the ship's head steady on numerous headings. This requires the vessel to be in open water and away from possible sources of magnetic interference such as cranes, steel piles reinforced concrete jetties, etc.

Some preliminary adjustments, based on a detailed analysis of compass deviation history (if available), may be made prior to sailing. Other adjustments, if made with the vessel alongside, will be at least partially based on guess work and cannot be relied upon until compass headings and any deviation has actually been observed on the compass.

Some adjusters will claim that, because of their "expertise", there is no need for them to go to the trouble of going to sea with the ship. Large discrepancies between actual deviation and that "predicted" by the adjuster, sometimes as much as 30 degrees, have been observed on compasses which have been "expertly adjusted" alongside. A valid deviation card cannot be issued unless compass headings have actually been observered and deviation recorded.

Swinging the compass of a trailer boat

There is nothing to say that a boat has to be on the water to have its compass swung and adjusted accurately. However, compass adjustment with the boat on a trailer is not widely accepted practice as it is difficult to determine, with absolute certainty, that there are no magnetic fields from the trailer, towing vehicle, surrounding structures, etc, affecting the compass.

Specialist instruments, such as a magnetometer, are required to determine whether any such fields are present. If they are, compass deviation observations will be inaccurate. Steel vessels definately cannot be swung in this way as it is impossible to differentiate between the magnetism of the hull and that of the trailer, etc.

The swing must be carried out on a level, open space such as a field, away from buildings, reinforced concrete, overhead cables, underground pipes, etc, using the same methods to determine deviation as for an on-the-water compass swing.

An advantage of swinging the compass this way is that the boat can be "parked" steady on each of the required headings whilst adjustments are made.

Why does a boat compass card sometimes tilt?

The earth's magnetic field travels from the Magnetic South Pole to the Magnetic North Pole. For mathamatical convenience it is divided into two major components: vertical and horizontal. The closer to the poles, the stronger the vertical component and the weaker the horizontal component. At the magnetic equator the horizontal component is at its strongest and the vertical component is zero.

The angle of the magnetic field in relation to the horizontal is known as MAGNETIC INCLINATION or MAGNETIC DIP .

In the south, the magnetic field comes up, out of the earth and in the north, it goes back down, into the earth. As the compass needle will naturally align itself with the earth's magnetic field and is integral with the card of a marine compass, the upwards or downwards magnetic force will cause the compass card to tilt. The closer to the poles, the stronger the upwards or downwards force and the greater the tilt. To counter this, the card of a boat compass usually has a small counter-weight attached to enable it to sit level.

A yacht or boat compass specifically designed for Northern Hemisphere use will have a weight positioned to counter the downward magnetic force. When this compass is brought to the Southern Hemisphere, the combination of the weight and the upwards magnetic force will create an exagerated tilt on the card. Obviously, the same thing will happen to a Southern Hemisphere compass when it goes to the north. Sometimes the tilt is so great the compass becomes inoperative.

Rebalancing the compass for the opposite hemisphere involves dismantling the compass and moving the weight to the opposite side of the card and is not usually considered economically viable or indeed practicable. For a yacht travelling between the higher latitudes of one hemisphere to the other, carrying two interchangable compasses, one balanced for each hemisphere might be advisable.

Other reasons for compass card tilt:

Heeling error magnets (if fitted) require adjustment
Damaged card float chamber
Damaged jewel pivot
Card dislodged from pivot
Low liquid / fluid level

Degaussing - A brief explanation

A gausse is a unit of measurement of the strength of a magnetic field. Therefore, by definition, to degausse is to reduce the strength of, or eliminate, the magnetic field.

Degaussing ships was first carried out during World War II as protection against magnetic mines. A magnetic mine is activated when the earth's magnetic field is distorted by the magnetism of a passing steel ship. Degaussing could be described as magnetic camouflage.

Three sets of electro-magnetic coils (for the vertical, longitudinal and athwartship axis) are installed on the ship and arranged in such a way that, when they are energised with a high amperage direct current , the ship's magnetic field is hopefully made indistinguishable from the earth's own magnetic field, thus considerably reducing the chances of triggering the magnetic mine. Each axis is treated seperately.

Ships not fitted with degaussing coils can be degaussed either by dragging a cable carrying around 2000 amps alongside the ship or, more efectively, by encircling the vessel with cable. Ships degaussed in this way require scheduled degaussing as the effects gradually wear off.

Degaussing cables were originally copper but much lighter, super-conducting, ceramic degaussing cables have now been developed.

Degaussing a vessel will cause significant deviation to the magnetic compass. The compass is compensated for degaussing by energising coils around the binnacle to create fields equal and opposite to those created by the degaussing coils. Each axis, vertical, longitudinal and athwartships, is treated seperately as in normal compass compensation.

For ships with permanently installed degaussing coils, two seperate magnetic compass deviation cards will be required: One for degaussing coils switched on and one for degaussing coils switched off .

Why engage a professional, qualified compass adjuster?

Effective correction, or compensation, of the marine compass for any deviation error found during the compass swing requires an understanding of the earth's and ship's magnetic fields and an ability to differentiate between the permanent magnetism of the ship's hard iron and the induced magnetism of the ship's soft iron .

It is necessary to recognise the effect the various magnetic fields have on the ship's compass and to have a practical knowlege of the workings of the marine compass and its correctors. Simply reducing or eliminating compass deviation on a vessel in one location can actually make it worse when the vessel travels to another location, particularly when substantial changes in latitude are involved.

Whilst amateur or DIY compass adjusting is not a completely outrageous concept on pleasure craft, it has been known to transform a relatively simple problem into a fairly complex one, particularly on steel vessels.

Most licensed compass adjusters are highly skilled technicians, professional seafarers and qualified navigators who have undertaken rigorous and comprehensive training to meet national and international standards.

National marine agencies specify that commercial vessels have their compass adjusted only by a person qualified and authorised to do so. International standards for magnetic compasses and compass adjusting are governed by the International Organization for Standardization (ISO) and the International Maritime Organization (IMO) SOLAS 74 Convention .

From time to time an air bubble may appear in the damping liquid in the bowl of a marine compass. This is often a result of leakage around the seals between the bowl and the diaphram or the glass. Sometimes it indicates damage to the bowl or diaphram. A small bubble will not in itself affect the performance of the compass but may partially obscure the compass card. A larger bubble can have an adverse effect on performance. Removing the bubble requires some patience as it is necessary to replace the air with liquid. Some modern, cheaper compasses are sealed units and cannot be refilled. If the compass is refillable and is leaking a lot of liquid, an attempt at repairing might be made before refilling. Often, particularly in the case of small cheaper compasses, purchasing a new compass is found to be the most economical option.

Time to buy a new compass?

Finding the correct liquid/fluid for the compass can be a problem. It can be one, or a mixture, of several ingredients. Different manufacturers use different ingredients and some are not compatible with others. Some are not compatible with the compass and can remove the paint and markings from the compass card or cause other damage. Some are oil based, some are water/spirit based. The safest option is to obtain the correct liquid from the manufacturer. Unfortunately this can be difficult. Some chandlers will stock "compass liquid" but the ingredients of this are often unknown. If the required ingredients can be determined, it may be possible to obtain suitable liquid from local sources, at a much cheaper rate.

To check compatibility, draw some existing liquid out of the bowl with a syringe and mix with a small amount of the new liquid. It will often be immediately obvious if it is not compatible. The following are some of the main types of compass liquid ingredients:

Ethyl alcohol (ethanol) / distilled water
Isopropanol (rubbing alcohol) / distilled water
Kerosene (paraffin oil)
Silicon oil
Mineral oil

Why is it necessary to swing the compass regularly? .
How long is a deviation card valid? .
Legislative requirements .

Over a period of time and after certain events, the addition or removal of equipment or the carriage of magnetic cargo such as iron ore, the vessel's magnetic fields may change, altering the residual deviation of the compass. In some circumstances the changes can be quite dramatic.

New steel vessels will have their compass adjusted when first commissioned. It has been known for a one or two year old vessel to record deviation as high as 30 to 40 degrees, as the residual magnetic fields created during the building process gradually dissipate.

Navigators of sea going vessels are required to observe and record compass error daily whilst on passage. These observations are important, not only for safe navigation, but also to assist the compass adjuster in making an accurate analysis of the causes of deviation, should the compass require adjustment.

International Organization for Standarazation (ISO) and the International Maritime Organization (IMO) SOLAS 74 Convention and HSC Code (for high speed craft) SOLAS CHAPTER V, REGULATION 19.2.1:

All ships irrespective of size shall have:

a properly adjusted standard magnetic compass, or other means, independent of any power supply, to determine the ship's heading and display the reading at the main steering position.
a pelorus or compass bearing device, or other means, independent of any power supply, to take bearings over an arc of the horizon of 360°".

ISO 25862 : 2009 (E): States that all SOLAS vessels should have their compass swung/adjusted and a new deviation card issued at maximum two yearly intervals. When a new vessel is commisioned, compass deviation on any heading should be no more than 3°. Thereafter, deviation on any heading should be 5° or less.

Vessels transiting the Panama Canal are required by the canal authorities to have had a valid compass deviation card issued within the previous 12 months.

Many maritime authorities and organisations stipulate that the magnetic compass is to be swung and adjusted annually. Prudent mariners and vessel operators will always ensure that the compass is regularly checked and properly adjusted.

► Hong Kong Marine Department magnetic compass requirements - click here
In Australia , any Non-SOLAS vessel operating under state survey (USL Code) is required to have its magnetic compass examined and adjusted by an approved compass adjuster at maximum three yearly intervals or maximum four yearly intervals if operating under the recently introduced NSCV . In addition to regular routine checking of the compass for deviation, and adjustment for survey compliance, all sea going vessels should have their compass inspected, swung and adjusted, and a new deviation card issued, when any of the following apply:

On a new vessel

After periods of lay up

When a new compass is installed

When deviation exceeds 5 degrees on any heading

After trauma, such as lightning strike, grounding, fire, etc

When compass performance is unsatisfactory or unreliable

When a record of compass deviation has not been maintained

After alterations & additions to vessel's structure & equipment

After repairs involving welding, cutting, grinding, etc which may affect the compass

When electrical or magnetic equipment close to the compass is added, removed or altered

When compass deviation does not appear to correspond with that shown on deviation card

When the validity period of the deviation card set by the national or state marine authority is due to expire