Handbook of Magnetic Compass Adjustment - Maritime Safety ...
Handbook of Magnetic Compass Adjustment - Maritime Safety ...
Handbook of Magnetic Compass Adjustment - Maritime Safety ...
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Substituting deviation data algebraically, east being plus and west minus,<br />
√2 1<br />
B = 8<br />
(20.0 – 1.2 + 8.0 + 6.8) + 4<br />
(11.5 + 12.5)<br />
B = +12<br />
√2 1<br />
C = 8<br />
(20.0 + 1.2 + 8.0 – 6.8) + 4<br />
(10.5 + 5.5)<br />
C = +8<br />
502. Reasons for analysis. This method <strong>of</strong> estimating approximate coefficients is convenient for:<br />
(1) Analyzing an original deviation curve in order to anticipate necessary corrections.<br />
(2) Analyzing a final deviation curve for the determination <strong>of</strong> additional refinements.<br />
(3) Simplifying the actual adjustment procedure by anticipating effects <strong>of</strong> certain corrector changes on the deviations at<br />
all other headings.<br />
503. Approximate and exact coefficients. It is emphasized that the above estimations are for the approximate coefficients<br />
and not for exact coefficients. Approximate coefficients are in terms <strong>of</strong> angular deviations that are caused by certain magnetic<br />
forces, and as stated before, some <strong>of</strong> these deviations are subject to change with changes in the directive force, H. The exact<br />
coefficients are expressions <strong>of</strong> magnetic forces, dealing with: (a) arrangements <strong>of</strong> s<strong>of</strong>t iron, (b) components <strong>of</strong> permanent<br />
magnetic fields, (c) components <strong>of</strong> the earth's magnetic field, and (d) the shielding factor λ. Thus, the exact coefficients are<br />
expressions <strong>of</strong> magnetic force which produce the deviations expressed by the approximate coefficients. The exact coefficients<br />
are for mathematical considerations, while the approximate coefficients are more practical for adjustment purposes. For this<br />
reason, the exact coefficients and the associated mathematics are not expanded further in this text.<br />
504. <strong>Compass</strong> heading and magnetic heading. When deviations are large, there is an appreciable difference in the deviation<br />
curve if it is plotted on cross-section paper against compass headings or against magnetic headings <strong>of</strong> the ship. Not only is<br />
there a difference in the shape <strong>of</strong> the curves, but if only one curve is available, navigators will find it difficult in applying<br />
deviations when converting from magnetic heading to compass heading, and vice versa. When deviations are small no<br />
conversion is necessary. Figure 504 illustrates the differences mentioned above by presenting the deviation values used in<br />
figure 501 as plotted against magnetic headings as well as against compass headings.<br />
Figure 504 – Comparison <strong>of</strong> deviation curves (magnetic heading vs. compass heading).<br />
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