Navigation definitions

The words for where you point and where you go.

North references

• Variation: the angle between true north and magnetic north at your position. The chart shows it with isogonic lines (isogonals), which join places of equal variation. The agonic line is where variation is zero, so true and magnetic north line up.
• Deviation: compass error from the aircraft's own metal and electrics. Read it off the deviation card by the compass. It changes with heading, so it is not one fixed number.

Variation is about the earth; deviation is about your aircraft. Do not swap them.

Directions each add one correction to the last:

• True: measured from true north.
• Magnetic: true adjusted for variation.
• Compass: magnetic adjusted for deviation.

Path and pointing

• Track: the path over the ground.
• Heading: where the nose points.
• Drift: the angle between heading and track that a crosswind opens up.
• Wind correction angle (WCA, or crab): the angle you turn the heading into wind to cancel drift and hold the track.
• Bearing: the direction of one point from another, measured from north.

Note: isogonic lines, the agonic line, and the deviation card are standard chart/cockpit conventions, not defined in the AC 100-001 glossary or TC AIM. Track, heading and bearing are glossary terms and drift is in TP 1102; variation and deviation (compass error) follow standard convention (the glossary's "deviation" means departing a clearance, not compass error).

Trap: calling the nose direction your "track". Heading is where you point; track is where you actually go, and wind sets them apart.

Practice