Compass error consists of the compass variation, the changes wrought on the accuracy of a compass by the presence of iron or steel around it, and compass deviation, the difference between magnetic north and true north. If one error is to the east and one error to the west, then one error will offset the other. Both are part of the process of establishing the true course of a vessel mathematically, when the end result is combined with the course on the ship's navigational compass.
Locate the compass deviation information on the nautical chart of the area in which you are operating.
Record the deviation. If the deviation is "East," put a plus sign in front of it. If the deviation is "West," put a minus sign in front of it.
Write the variation --- found on the compass card, located near the compass. As with the deviation, it will indicate "West" or "East" variation by the letters "E" or "W" following the number of degrees of variation. If the variation is "East," put a plus sign in front of it. If "West," put a minus sign in front of it.
Add the numbers, relative to the signs before them. For example, if the variation is 9 degrees west and the deviation is 4 degrees east, minus 9 plus 4 equals minus 5, the total compass error.