What does the term "magnetic variation" refer to in navigation?

Magnetic variation refers to the angle between true north and magnetic north. This angle is crucial for navigators because it indicates the difference between the direction that a compass points (which is magnetic north) and the actual geographic north pole (true north).

Understanding magnetic variation is vital for accurate navigation, as it allows pilots and navigators to correct their compass readings to ensure they are heading in the correct direction. The value of magnetic variation can change depending on your geographical location and can also change over time due to changes in the Earth's magnetic field.

The other options relate to different aspects of navigation and performance rather than magnetic variation. For instance, true airspeed affected by wind is an important factor in flight planning, but it does not relate to the magnetic relationships that define magnetic variation. Similarly, discrepancies in altitude readings and differences in ground speed also involve different navigational principles and calculations rather than the magnetic north and true north relationship.