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Compass Deviation Analysis Explained

Compass deviation data usually have a very characteristic pattern, which can be analyzed mathematically into various components, or Coefficients. Compass analyis is useful because each of the Coefficients (or more precisely, 'Apparent Coefficients') is associated with a certain type of problem, and each coefficient can be addressed individually. Knowing the value of the Coefficients can simplify the process of compass adjustment and can help explain the source or cause of compass deviations that cannot be corrected.

How to determine the Apparent Coefficients is discussed in some detail in The American Practical Navigator HO 9, Chapter 6 (Bowditch). Beginning at page 14.

Here is a quick course:

Fig 1. shows a ship with a compass deviation of 2.5 degrees caused by athwartship permanent magnetism, which produces Apparent Coefficient C. Note the distinctive sine wave pattern of the graph. Fig 1.

Typically, compass deviation is analyzed into 5 Coefficients: A, B, C, D & E. And the total deviation is the sum of all the coefficients (see Figure 2).

Separating the Coefficients using Analysis will suggest a source of each component, as well as a possible way to correct it.

Coefficient A: is a constant error which is the same on all headings

Coefficient B: varies as a function of the sine of the compass heading, and

Coefficient C: varies as a function of the cosine of the compass heading, and

Coefficient D: varies as a function of the sine of twice the compass heading, and

Coefficient E: varies as a function of the cosine of twice the compass heading, and

Try experimenting with the Compass Deviation Analysis Calculator.