How to Perform Compass Calculations
The following table will allow you to visualize the steps necessary to
perform compass calculations. The table is followed by several
example exercises that should be completed before you move on to
Write down the first letters from the
phrase "Can Dead Men Vote Twice."
Ordered course is 180°T; you want to
find the compass course to steer. You
already know what T is, so write it
Lets say that the corrected variation
from the center of the compass rose is
When uncorrecting, remember that
you add westerly errors and subtract
easterly errors. The variation is
easterly, so subtract it from the true
heading to find the magnetic heading.
Next, from the deviation table, figure
2-8, find the value closest to 169°,
interpolating as necessary; write it
Remember, when uncorrecting you
add westerly errors. 15 + 169 = 184.
Now you can see that to head 180° true, you must steer 184° by this
particular magnetic compass. In this example, we were uncorrecting
(changing from true to compass). We could have used the same method
to change from compass to true; but we must remember that when
correcting, we add easterly and subtract westerly errors. With an
understanding of these rules, we can now go on to applying the lessons
learned to a functional part of a Quartermasters jobrecording entries
in the Magnetic Compass Record Book. But first, complete the training
examples on the next page.