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Charts and magnetism
Mercator preserves shape and rhumb lines but distorts scale with latitude; Lambert conformal conic is common on topographical charts. Variation is the angle between true and magnetic north (isogonals join equal variation); deviation is the aircraft compass error. Variation west, magnetic best (add); variation east, magnetic least (subtract).
Dead reckoning
True heading = true track − drift; magnetic heading = true heading ± variation; compass heading = magnetic heading ± deviation. Groundspeed = distance ÷ time.
Aids overview
NDB/ADF gives a bearing, VOR a magnetic radial, DME slant range, ILS lateral and vertical guidance, and GNSS position for RNAV.
Worked example
True track 090°, variation 10°W, deviation 3°E, no wind. Magnetic track = 090 + 10 = 100°. Compass = 100 − 3 = 097°. If the wind gives 5° right drift, the true heading becomes 085°, magnetic 095°, compass 092°.
Traps
Applying deviation when variation is meant; adding the wrong sign for west variation; confusing track with heading; forgetting that VOR radials are magnetic while the chart graticule may be true.