From BoatUS “Ask the Experts” ~ Navigation
If I follow a compass
course from my home port to a waypoint, why is there not always a 180 degree difference between that course and the compass
course that I follow to return home? It usually varies by a few degrees. Shouldn't the two courses it be exactly 180 degrees apart?
There are two forces that cause errors in your magnetic compass. One is called Variation - the angular difference between True North and Magnetic North. The other is called Deviation - the error in your compass caused by onboard electro-mechanical equipment
, and in some cases by the vessel itself.
The question involves what we call "reciprocal" courses. That is, when you travel one course from point "A" to point "B" and another from point "B" to point "A" in exactly the opposite direction as your original course. The interference
that causes deviation in a magnetic compass is sometimes between your magnetic compass and the North pole and sometime it is not. So, as you change your course, you change the amount of deviation.
If your course to your first waypoint is 261 degrees magnetic and the reciprocal course back to your original departure point is 078 degrees magnetic, there is a 183 degree difference. The additional 3 degrees is due to the difference in deviation experienced on both courses.