General Navigation
General Navigation tests position, direction, distance, time and chart work, including the differences between great-circle and rhumb-line tracks and the conversion between true and magnetic directions.
Core rules
- Latitude is measured north/south of the equator; longitude is measured east/west of the prime meridian.
- One minute of latitude equals one nautical mile.
- A rhumb line cuts all meridians at the same angle; a great circle is the shortest distance but changes track.
- Variation is the angle between true and magnetic north; deviation is the error of the aircraft compass.
- “Variation east, magnetic least”: to convert true to magnetic, subtract easterly variation and add westerly variation.
Worked example
From 15°N 120°E to 15°N 123°E, the rhumb-line distance along the parallel is 3° × 60 × cos 15° = 180 × 0.966 = 173.9 NM. The great-circle distance is slightly shorter, but the rhumb line keeps a constant track of 090°T, which simplifies navigation.
Traps
- Applying the latitude scale to longitude measurements.
- Forgetting to apply variation before deviation, or using the wrong sign.
- Treating a great-circle track as constant.
- Mixing UTC and local time in ETA calculations.