There are several sets of "memory-aids" available to assist you in
performing compass calculations. The following simple phrases are
designed to assist you in remembering how to convert from one heading
expression to another and how to name errors.
The first set of aids were designed to help you remember the
arrangement of the first letters of each word in the phrase.
arranged representing the three ways of naming a direction (compass,
magnetic, true) with the respective differences (deviation and variation)
properly placed between them: (CDMVT) compass, deviation,
magnetic, variation, true. The first letters in the words at elections stand
for add east (subtract west), when converting the direction from compass
to true. When converting in the opposite direction, the letters are
reversed (TVMDC) and the memory aid "timely vessels make distance
count at war" informs us to add west (subtract east) error when
converting from true to compass.
The second set of memory aids deal with comparison of two compass
headings to determine whether to call the difference east or west. If the
comparison is between magnetic and compass, and compass is a greater
number (best), the difference is west. The same comparison can be
made between true and magnetic. In this case, magnetic is considered
the same as compass.
Correcting: converting from compass course to a true course
Uncorrecting: converting from a true course to a compass course