convert
magnetic into true bearings using declination
(1.5 degrees West in central Wisconsin)
The agonic line (of 0� declination) currently passes through
Superior and Prairie du Chien, Wisconsin. It is currently drifting
westward approximately 10' of longitude, or about 4 miles (6.5 km), per
year.
If you are west of the agonic line,
the declination is toward the east (positive); if east of the 0�
line, the declination is west (negative).
Where the magnetic declination is zero
(at the agonic line), a compass needle points
toward true north.
To translate the difference between the geographic and magnetic
meridian, you must algebraically subtract the angle of
declination from the magnetic north reading. To correct a magnetic compass reading to true north when west of the 0�
line, the declination value should be subtracted from the magnetic north
reading.
Stevens Point,
however, is east of the 0� line and has a magnetic declination of approximately 1�
44' west (calculation
in 2003). You should still subtract the
declination from the magnetic north reading, but remember that in this
case the declination value is negative (subtracting a negative number essentially
means adding the declination value to the magnetic
north reading). Therefore, a magnetic bearing of 353�Mag =
351.5�True at Stevens Point. (click the picture
for a fullsize view; use the Back button to return)
