Handbook of Magnetic Compass Adjustment - Maritime Safety ...

413. Study of adjustment procedure. Inspection of the general B, C, and D combination of errors, pictured in figure 413 will
reveal that there is a definite isolation of the deviation effects on cardinal compass headings.
Figure 413 – B, C, and D deviation effects
For example, on 090° or 270° compass headings, the only deviation which is effective is that due to B. This isolation, and
the fact that the B effect is greatest on these two headings, make these headings convenient for B correction. Correction of the
B deviation on a 090° heading will correct the B deviation on the 270° heading by the same amount but in the opposite
direction and naturally, it will not change the deviations on the 000° and 180° headings, except where B errors are large.
However, the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090° or
270° deviation correction, but only by seven-tenths (0.7) of that amount, since the sine of 45° equals 0.707.
The same convenient isolation of effects and corrections of C error will also change the deviations on all the intercardinal
headings by the seven-tenths rule, as before. It will now be observed that only after correcting the B and C errors on the
cardinal headings, and consequently their proportional values of the total curve on the intercardinal headings, can the D error
be observed separately on any of the intercardinal headings. The D error may then be corrected by use of the spheres on any
intercardinal heading. Correcting D error will, as a rule, change the deviations on the intercardinal headings only and not on
the cardinal headings. Only when the D error is excessive, the spheres are magnetized, or the permanent magnet correctors
are so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as a
result of sphere adjustments. Although sphere correction does not generally correct deviations on cardinal headings, it does
improve the stability of the compass on these headings.
414. If it were not for the occasional A or E errors which exist, the above procedure of adjustment would be quite sufficient,
i.e., adjust observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal heading.
However, figure 414, showing a combination of A and B errors, will illustrate why adjusting procedure must include
correcting deviations on more than the three essential headings.
If the assumption were made that no A error existed in the curve illustrated in figure 414, and the total deviation of 6°E on
the 090° heading were corrected with B magnets, the error on the 270° heading would be 4°E due to B overcorrection. If then,
this 4°E error were taken out on the 270° heading, the error on the 090° heading would then be 4°E due to B undercorrection.
Figure 414 – A and B deviation effects
17