DanNautic

It will be fair to inform and warn navigators about unstable Magnetic Compass readings when vessels are trading in height latitudes. Those warnings are stated below

In high magnetic latitudes particularly in arctic waters where the Horizontal Intensity (H) of the Earth’s Magnetic Field is weak (H is proportional to cosine to the angle of the Inclination (I) of the Earth’s Magnetic Field) the Strength of the Magnetic Force (K) from the Vessel can be of great significance to the Total Magnetic Force (H’) on the Magnetic Compass that is H’ could be very weak and

• difficulties could arise for the Magnetic Compass to settle to a steady course
• the deviation could be dominated mostly by the Strength of the Magnetic Force (K) from the Vessel

or in worst case

• the Magnetic North of the Magnetic Compass could be kept in the direction of  the Strength of the Magnetic Force from the Vessel (K) whatever heading the vessel may have.

Please also note.

In high magnetic latitudes where H’ could be very weak (ref. the notice above) the vessel must be steady* on the course

• before you compare the Magnetic Compass Course to the Gyro Compass Course
• before you make a Deviation Control
• before you make a Magnetic Compass Adjustment

*It may take quite a long time – up to a few minutes – before the Magnetic Compass Course becomes steady.