True vs. Magnetic vs. Compass Heading — Variation and Deviation Explained

Every cross-country nav log walks the same chain of corrections from a line drawn on a chart to the number you actually hold on the compass. It's simple once it clicks, but the signs are easy to get backwards — and a sign error on variation in the western U.S. can put you 20–30 degrees off course. This guide explains each step, which aviation sources use true or magnetic references, and works a complete example.

The Chain of Corrections

• True course (TC) is the line you draw on the chart, measured from true north at the meridian nearest the middle of the leg.
• Wind correction angle (WCA) turns course into heading. Wind from the right means a correction to the right (add); from the left, subtract. See the E6B wind correction guide for how to compute it.
• Variation (VAR) is the angle between true north and magnetic north at your location.
• Deviation (DEV) is the error in your specific airplane's compass caused by its own magnetic fields — engine, radios, wiring.

Magnetic Variation

Variation is printed on sectional charts as dashed magenta isogonic lines, labeled with a value such as "12°E." Along the agonic line, variation is zero. In the contiguous U.S. variation is generally east in the West and west in the East.

The memory aid for converting true to magnetic is "East is least, West is best": subtract easterly variation, add westerly variation. Going the other way, from magnetic to true, reverse it.

Compass Deviation

Every certificated airplane with a magnetic compass has a compass correction card near the compass. It usually reads like "For 090, steer 093," listing a magnetic heading and the compass reading needed to fly it. Deviation is normally a few degrees and varies with heading — and sometimes with radios or avionics turned on or off.

In the CockpitCalc converter, deviation is entered as magnetic minus compass. For "For 090, steer 093," that's 090 − 093 = −3. For course work that doesn't involve the airplane's compass — plotting, VOR radials, airway courses — leave deviation at zero.

Worked Example

You plot a true course of 090°. Winds aloft give a wind correction of 8° to the left. The sectional shows 12°E variation, and the compass card says "For 070, steer 072."

• True heading: 090 − 8 = 082°
• Magnetic heading: 082 − 12 (east is least) = 070°
• Compass heading: the card says steer 072° (deviation entered as −2)

Get the variation sign backwards and you'd hold 094 instead of 070 — a 24° error that puts you about 4 NM off course for every 10 NM flown.

Which Sources Use True, Which Use Magnetic?

• True: courses measured on sectional and WAC charts, METAR and TAF wind directions, winds aloft forecasts, and area forecasts.
• Magnetic: runway numbers, ATIS/AWOS/ASOS broadcast winds and winds given by the tower, VOR radials, airway courses, and headings assigned by ATC.

That split explains a common surprise: the METAR wind looks like a direct crosswind, but the ATIS wind is nearly down the runway. With 12–14° of variation, the two can differ noticeably. When you compute a crosswind component, convert so the wind and runway use the same reference.

Using the CockpitCalc True ↔ Magnetic Converter

1. Enter variation from your chart: positive for East, negative for West.
2. Enter deviation from the compass card as magnetic minus compass, or zero for course work.
3. Choose True → Magnetic, Magnetic → True, or Compass → True, and enter the heading.