Basic Navigation Principles and the Earth
Earth model and coordinates
The Basic Navigation Principles and the Earth domain tests the foundation for every later calculation: the Earth’s shape, coordinate systems, direction, magnetism and time. Treat the Earth as an oblate spheroid. A great circle is the shortest surface path between two points; a rhumb line cuts every meridian at the same angle and is easier to steer but longer. Latitude is measured north or south of the equator from 0° to 90°; longitude is measured east or west of the prime meridian from 0° to 180°. One minute of latitude is one nautical mile, while one minute of longitude shrinks with latitude.
Direction and magnetism
True direction is measured from true north; magnetic direction is measured from magnetic north. Variation is the angle between them, shown on isogonals, and is named east or west. Deviation is aircraft-specific, caused by electrical and magnetic fields, and is read from the compass card. Use the sequence True–Variation–Magnetic–Deviation–Compass. From true to magnetic, add west variation and subtract east variation. From magnetic to compass, subtract east deviation and add west deviation. Time is expressed in UTC (Z) to avoid zone confusion.
Worked example
True course 090°, variation 8°W, deviation 2°E. True to magnetic: west variation is added, giving 098°M. Magnetic to compass: east deviation is subtracted, giving 096°C. The aircraft is steered 096° compass to make good 090° true, ignoring wind.
Traps
Do not mix true and magnetic values on the same log. Check whether a chart or forecast gives variation. Remember that one minute of longitude is not one nautical mile except at the equator. Easterly variation is subtracted when going from true to magnetic; reversing that rule costs marks.