What is compass error?
Compass error is the difference between true north and the heading shown by the magnetic compass. It is the sum of variation and deviation. Variation is the difference between true north and magnetic north at that place, taken from the chart. Deviation is the extra error caused by the ship’s own magnetism and changes with heading.
How this calculator works
East variation or deviation is treated as positive. West is treated as negative. Compass Error = Variation + Deviation. If a true heading is entered, Compass Heading = True Heading − Compass Error, then normalised to 0–360°. The memory aid “Error East, Compass Least / Error West, Compass Best” is built into this sign convention.
Who should use it
Useful for bridge officers taking azimuths and writing the compass error book. Always compare with a gyro or a celestial azimuth when possible. This tool does not replace a proper compass swing.