Gyro & Compass Error Tool True, Magnetic, Compass & Bearing Correction

Gyro Error • Magnetic Variation • Compass Deviation • True / Magnetic / Compass / Gyro • Known-Bearing Observations • Deviation Card • Relative Bearings

Gyro & Compass Error Tool True, Magnetic, Compass & Bearing Correction

Convert courses, headings and bearings among True, Magnetic, Compass and Gyro references using an explicit east-positive / west-negative convention. Simple mode gives a fast four-reference conversion and relative-bearing correction. Advanced updates chart variation by annual change, derives gyro error and magnetic-compass deviation from multiple known-bearing observations, interpolates a deviation card by heading, compares manual and observed error sources, and builds a bridge correction ledger with scatter and outlier diagnostics.

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Need observed gyro error, deviation-card interpolation or annual variation update? Click Advanced above.Advanced turns this into a bridge compass-check workflow with multiple observations and separate error sources.

Fast6 primary inputs

ConventionEast + / West −

OutputsTrue, Magnetic, Compass & Gyro

Four-Reference Course / Bearing Conversion

Entered angle0–360°.

Entered reference

Magnetic variation / declinationdegrees: East positive, West negative.

Magnetic compass deviationdegrees: East positive, West negative.

Gyro errordegrees: East positive, West negative. True = Gyro + error.

Relative bearing from bow000° ahead, 090° starboard, 180° astern, 270° port. Uses converted True angle as ship heading.

Magnetic Variation Update

Use chart/WMM data in signed degrees. NOAA uses east declination positive and west negative; annual change is also entered east-positive.

Variation source

Variation at epoch° E positive / W negative.

Epoch year

Annual change°/year, eastward positive.

Assessment year

Manual current variation

Error Source & Operational Conversion

Gyro-error source

Manual gyro errorE + / W −.

Observation residual review thresholddegrees from mean.

Deviation source

Manual deviationE + / W −.

Compass heading for deviation lookup°C, deviation depends on ship heading.

Primary entered anglecourse / heading / bearing to convert.

Primary reference

Relative bearing from bowClockwise 000–360°.

Known True-Bearing Observation Register

Use a charted range/transit, calculated celestial azimuth/amplitude or another independently known true bearing. Each row can derive gyro error and magnetic-compass deviation. Blank gyro or compass readings are ignored for that sensor.

Ship compass heading °C	Known true bearing °T	Observed gyro bearing °G	Observed compass bearing °C	Method

Magnetic Compass Deviation Card

Enter signed deviation by compass heading. The tool circularly interpolates between surrounding headings, including the 315°→000° sector.

Compass heading °C	Deviation ° (E + / W −)

Bridge Compass / Heading Checks

Variation source/date verifiedChart/WMM declination and annual change apply to the vessel’s location/date. Deviation card/current table verifiedMagnetic compass deviation data are current for vessel condition/equipment state. Gyro settled / operating normallyAny manufacturer latitude/speed inputs and alarms are checked. Known-bearing source verifiedTransit/chart/celestial true bearing is independently valid. Repeater alignment checkedObserved bearing source/repeater alignment is understood. Multiple observations comparedScatter and changing conditions are reviewed rather than using one unexplained value. True / magnetic / gyro references labeledBridge team and passage plan use unambiguous reference suffixes. Compass errors logged / applied per procedureShip-specific SMS/watchkeeping requirements remain controlling.

Navigation limitation: this tool performs reference conversion and consistency screening. It does not replace compass adjustment, manufacturer gyro corrections, approved deviation cards, celestial/navigation publications, bridge procedures or watchkeeper judgment. Magnetic variation changes by position and time; magnetic deviation changes with ship heading and onboard magnetic conditions; gyro error can contain fixed and dynamic components. Always label bearings/headings by reference.

Navigation basis: NOAA defines magnetic declination/variation as positive east and negative west and states that true bearing is obtained by adding declination to magnetic bearing. Bowditch describes gyro error in degrees east or west and notes that gyrocompasses are independent of magnetic variation/deviation. USCG exam guidance uses gyro error = true bearing − gyro bearing, with easterly error added to gyro to obtain true. IMO A.424(XI) defines true heading from true north clockwise through 360° and defines gyro-compass errors relative to true heading/settle point.

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