Magnetic Variation, Compass Deviation & TVMDC Calculator
Convert True Heading to Magnetic and Compass Headings with 'East is Least, West is Best' and Solar Kp index.
Last reviewed by the myCalculator Editorial Team
Input Parameters
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Formula & How Magnetic Variation, Compass Deviation & TVMDC Calculator Works
A magnetic compass points to magnetic north, which differs from true north by the local variation. To convert, Magnetic = True ± Variation: use the rule 'east is least, west is best', meaning subtract easterly variation and add westerly variation. A true track of 090° with 10°W variation is a magnetic track of 100°. The compass heading then also depends on deviation, the error caused by the aircraft's or boat's own magnetic fields, which is applied in the same way and is listed on the compass card. Variation changes by location and slowly over time, so use a current chart or the aviation publications. Always cross-check against the aircraft's approved procedures.
- Enter True Heading (Track/Route °): Input your target parameter value.
- Enter Magnetic Variation (°): Input your target parameter value.
- Enter Variation Direction (E/W): Input your target parameter value.
- Enter Compass Deviation (°): Input your target parameter value.
- Enter Deviation Direction (E/W): Input your target parameter value.
- Enter Solar Space Weather Kp Index (0-9): Input your target parameter value.
- Apply Formula: Calculate using
Magnetic = True ± Variation | Compass = Magnetic ± Deviation ('East is Least, West is Best'). - Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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Frequently Asked Questions (FAQs)
What does 'East is Least, West is Best' mean in navigation?
When converting from True to Magnetic: subtract Easterly variation (East is Least) and add Westerly variation (West is Best).
How do solar flares affect compasses and GPS?
Coronal mass ejections cause geomagnetic storms (Kp 5-9) that disturb Earth's magnetic field, causing compass jitter, radio blackouts, and GPS positioning errors.
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Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
CASR Part 91 general operating and flight rules, Part 121 fuel reserves, and flight crew licensing standards.
Aviation weather services, standard atmosphere (ISA) profiles, METAR/TAF aerodrome weather reports.
Aeronautical Information Publication (AIP), En Route Supplement Australia (ERSA), and airspace procedures.
CASA Navigation theory practice for RPL, PPL and CPL students, including compass, variation, track, wind and flight-planning topics.
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Operational Scenario
Operational Scenario: Coastal VFR Passage Approaching Deteriorating Stratus
You are planning a day VFR flight from Moorabbin (YMMB) to Latrobe Valley (YLTV) via the coastal visual corridor. The GAF indicates broken stratocumulus at 2,500 ft with localised coastal drizzle lowering the cloud base to 1,000 ft AMSL. The forecast freezing level is 4,500 ft with moderate mountain waves over the Strzelecki Ranges.
Practical Decisions
- •Compare the GAF area by area against your personal minimums, and decide before departure the cloud base at which you will turn back.
- •Check whether an alternate is required (CASR Part 91 MOS 8.04; AIP ENR 1.1 para 10.7.2): for an aeroplane, if you will arrive during, or up to 30 minutes before, forecast cloud of more than SCT below 1,500 ft or visibility below 8 km at the destination, you must provide for a suitable alternate — unless flying day VFR within 50 NM of the departure point.
- •Know what CAVOK means (AIP GEN 3.5): visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no cumulonimbus or towering cumulus, and no significant weather.
- •Set a fixed turn-back point before terrain traps or coastal cloud banks, and brief it as a decision already made.
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