Simrad Robertson AP9 Mk3, Instruction Manual, Page: 95 / 202

Installation
20169223D
5-5
magnetic influences such as the engines, cables, transmitter antennas
or other electromagnetic objects.
Note !
The compass must be adjusted.
Heeling error
Heeling error may be observed when the boat is rolling and pitching,
causing an unstable compass card. This can be adjusted for by using
a "heeling magnet", placed vertically below or above the exact centre
of the compass. The magnet is normally placed with the red end up
in the Northern Hemisphere and the blue end up in the Southern
Hemisphere. The correct distance between magnet and compass can
best be found during sea trials. Mounting the compass close to the
vessel’s centre of roll can also reduce the heeling error.
Northerly/southerly turning error
Symptoms of northerly turning error are that a vessel is "S-ing" on
northerly headings when at high latitudes. The reason for this
phenomena is that the earth's magnetic lines of flux are parallel to the
earth's surface only at the equator, and thus no vertical magnetic
component exists. When moving further north from the equator, the
vertical component of the earth's magnetic field increases.
The directional reading from a magnetic compass is based upon the
horizontal component of the earth's magnetic field. This component
becomes smaller and smaller with increasing latitude, while the
vertical magnetic component increases. The resulting effect at high
latitudes magnetic compasses become sluggish and appear to be
unstable. These symptoms become more apparent as speed increases.
The same phenomena is experienced in the southern hemisphere, but
on southerly courses and is referred to as southerly turning error.
There is no patent cure for this problem, besides making a proper
installation and adjustment of the compass.