Simrad AP28 Operator's Manual page 87

Lubber line
In certain areas and at high latitudes the local magnetic
interference becomes more significant and heading errors
exceeding ±3° may have to be accepted.
Mounting Offset
After compass calibration, the difference between the
compass lubber line and the boat's center line should be
compensated for.
1 Find the bearing from the boat position to a visible
object. Use a chart or a chart plotter
2 Steer the boat so that the center line of the boat is
aligned with the bearing line pointing towards the
object
3 Change the offset parameter so that the bearing to
the object and the compass readout becomes equal
Make sure that both the compass heading and the
bearing to the object have the same unit (°M or °T).
4 Confirm the Offset by pressing the MENU key
Lubber line
Setup at installation | 87
Magnitude of
local field in % of
earth's magnetic
field.
Direction of local
field with respect
to lubber line. It
can also be on
the reciprocal.