➣COMPASS DEVIATION ANALYSIS

Compass deviation is the difference between the vessel’s compass heading (what the compass says) and the vessel’s magnetic heading (what the compass should say). Note that true north and magnetic north are not the same: local variation must be taken into account. A compass reading high is said to have westerly deviation. Low, easterly. The same convention that applies for gyro error.

Compass deviation varies with heading. For example, a compass could read dead-on correct when pointing north, but read 50 degrees off when the vessel points west. This doesn’t mean anything is wrong with the compass itself. Deviation is a result of interfering magnetic forces caused by steel in the vessel and nearby electronics.

The adjuster’s job is to compensate for those interfering forces, balancing out the field such that the compass behaves as if the vessel was not there and indicates accurately.

Compass deviation follows a compound wave pattern: a complex wave that is the sum of simple sine and cosine waves. This pattern can be analyzed and broken down into its constituent components, called the five Apparent Coefficients. Each Coefficient is associated with a certain type of magnetic interference with a certain pattern, and each type is corrected individually. Knowing the Coefficients makes for extremely efficient and scientifically accurate adjustments.

The Five Coefficients

Compass deviation is typically divided into five Coefficients: A, B, C, D, & E. These Coefficients are the amplitudes of each wave that, added together, make up the complex deviation wave. This is described by the above equation, where phi is the compass heading. For mathematical purposes, easterly deviation is considered positive and westerly deviation is considered negative.

Coefficient A is a constant error that is the same magnitude and sign (+/-) on all headings. This can arise from several sources. 1) The compass is not installed with its lubber line parallel to the fore/aft axis of the vessel—this is corrected by rotating the compass. 2) The compass card is defective (magnetic axis of the card is not lined up with the numbers on the card)—requires a repair or a new compass. 3) The heading reference used to swing ship has a constant error (i.e. gyro error, or incorrect variation used)—if this can be verified, the A error can be disregarded. 4) In rare cases, A error can be caused by a specific orientation of soft iron (induced magnetism).

Coefficient B varies as a function of the sine of the compass heading. B error has its maximum influence on East and on West. It is associated with fore/aft permanent magnetism in the vessel, and is corrected with fore/aft magnets.

Coefficient C varies as a function of the cosine of the compass heading. C error has its maximum influence on North and South. It is associated with athwartships permanent magnetism, and is corrected with athwartships magnets.

Coefficient D varies as a function of the sine of twice the compass heading. D error is associated with induced magnetism caused by symmetrical soft iron: magnetism induced in the vessel by the Earth’s magnetic field, the force and direction of which varies with the vessel’s heading. It is often corrected with quadrantal spheres.

Coefficient E varies as a function of the cosine of twice the compass heading. E error is associated with induced magnetism caused by asymmetrical soft iron (imagine a compass off-center, as it would be installed on an aircraft carrier). It is corrected by either slewing the quadrantal spheres, or by installing additional quadrantal spheres at a 45 degree angle to the fore/aft axis of the vessel.

The graph above simulates deviation curves based on magnitude and sign of coefficients (the different error types) selected by the sliders. As in the above example, the y axis is compass deviation, and the x axis is compass heading.

Other Types of Deviation

Vertical interference, sometimes called Coefficient J, is corrected with a vertical magnet directly underneath the compass called a heeling magnet. This corrects for heeling error, which is the result of the ship’s vertical magnetism manifesting itself in the horizontal plane when the ship rolls, thus deflecting the compass, and making its heading indication unsteady.

Transient Deviations can occur in situations where electronics or DC current (or even AC current, if it is intermittent) can deflect the compass temporarily. One common example is a windshield wiper motor installed on the window near the compass. There are not always solutions to these issues, but once they are known, the pilot can beware. Also, a poorly adjusted compass (one that indicates incorrectly) will suffer significantly larger twitches from sources of transient magnetism than an adjusted one. An adjusted compass, although not impervious to transient deviations, is much more robust in its heading indication when faced with such problems.

The Flinders Bar is a vertical cylinder of soft iron commonly mounted in front of the compass, which corrects for the horizontal effect of vertical induction in the ship’s soft iron—an effect which varies with latitude. Or to be more precise, the effect varies with the angle of the Earth’s magnetic field, also known as the Dip Angle, which changes drastically depending on your latitude. This type of error has its maximum influence on East and West, and is called Induced B error. If observations of compass deviation on East and West are taken at both a high latitude and a low latitude, math can be employed to Split B: determine how much B correction is needed from B magnets (fore/aft magnets) and how much from the soft iron of the Flinders bar. When these are properly balanced, compass deviation will not change with latitude.

For further reading on methods of determining the values of the Apparent Coefficients, see the American Practical Navigator (Bowditch), Chapter 6. An excellent overview of the topic can also be found in the Handbook of Magnetic Compass Adjustment.

Footnotes to the above…

• The Apparent Coefficients are values which are relative to the strength of the interfering forces and the strength of the Earth’s field at the compass position. This is convenient: if B=2, that means 2 degrees of B error is in the deviation pattern. But there are also Exact Coefficients, whose values are the actual magnetic force being exerted on the compass needle. These are difficult to determine, and for practical purposes, they never need to be considered.

• Deviation as described by the above equation is a function of the compass heading, NOT a function of the magnetic heading (because the effect of interfering forces depends on the orientation of the compass needle with respect to the ship). When deviations are small, this distinction is negligible. When deviations exceed 6 degrees, the distinction becomes significant.

• If compass deviations exceed thirty or forty degrees, the coefficients begin to influence one another on a scale that wrecks traditional analysis. At this scale, the compass could easily be stuck on one heading, be completely unsteady, or behave so strangely that the data gathered from a swing is useless. This is common on brand new steel vessels. Often some examination with a magnetometer is required to determine the initial adjustments to make, before the compass begins to function and becomes susceptible to a traditional adjustment.