Compass Deviation

A magnetic compass needle’s movement is based upon the earth’s magnetic field.  That magnetic field and the movement of the magnetic needle on its pivot point in part, determine direction.

Ideally, there is no interference from any objects.

According to the book Staying Found by June Fleming:

“Rarely, the compass will be thrown off by something you can’t see: a local magnetic disturbance or a deposit of metallic ore…”

Such interference is called deviation.

Nickel near the earth’s surface, rifle barrels and power lines all interfere with the compass’ movement.  What are the standoff distances for deviation that the hiker need observe?

I went to two publications to get that answer.  Camping and Wilderness Survival by Paul Tawrell (page 177) and GPS Land Navigation by Michael Ferguson (page 53) became my sources.

Metal Object	Ferguson	Tawrell
Power Lines	55-60 meters	55 meters
Truck/Car	18-20 meters	10 meters
Telephone wires/barbed wire	8-10 meters	10 meters
Rifle/hand tools	2-3 meters	-
Pocket knife/binoculars/electronics	½ - 1 meter	-
Shovel	2-3 meters	½ meter

Interesting data points to consider.  For example, if you combine the bottom three rows of information it becomes common sense to ensure the hiker’s gear is out of the way.

Neither reference identifies the amount of impact these objects will have on the compass needle.  That is not practical for the average hiker.  It is the hiker’s movement away from the object (e.g., fence or car) to mitigate the error induced.

I would also offer that one should be careful how close to a compass electronics are stored in a pack.  I’d just keep it simple by stowing the flash light, GPS receiver and camera in a pack pocket/compartment away from the compass.

Note that GPS manufacturer Garmin recommends moving away from metal objects  when calibrating the GPS receiver’s electronic compass.