Does the north indicated by compasses point to north geographically? Is it affected by magnetic fields? How is the direction determined according to deviations when it is affected by magnetic fields? Why use magnetic headings? Let’s examine.
TN True North: It is the line drawn from any point on the earth to the north pole. All longitude lines are true north lines. They are fixed, they do not change.
MN Magnetic North: The north that the compass needle shows only if it is under the magnetic effect of the ground. Magnetic north is variable depending on region and time. (For now, somewhere over Canada, moving towards Russia.)
CN Compass North: It is the north indicated by the compass needle as a result of being affected by the magnetic fields of the surrounding devices other than the magnetic effect of the earth.
The north referred to in aviation is MN Magnetic North. Heading is determined by magnetic north.
Variation: There is little difference between the magnetic North-South direction, ie the heading of the compass needle only if it is under the magnetic influence of the ground, and the Geographical North-South direction. This difference is called Variation.
Deviation: A compass is affected by the magnetic fields of the surrounding devices except the magnetic effect of the place and shows a different direction from the magnetic field.
The angle between magnetic north and the true north indicated by the compass is called deviation.
If the goal is only to get magnetic heading, it will be sufficient to just look at the compass. On the other hand, the compass is read to measure True heading, then deviations are added.
For example, Magnetic heading 140 W and 5 degrees east magnetic deviation so true course 145 or Magnetic heading 140 W and 5 degrees west magnetic deviation so true course 135.