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