Compass Error and Deviation Table by Sun and Stars

Ship's time with the time zone number, as in the logbook, east positive: zone +3 means UTC is three hours behind ship's time. (A zone description in the Nautical Almanac carries the opposite sign.) The tool converts it to UTC and shows the result under the field. Enter seconds for accuracy: when the azimuth changes fast, half a minute matters.

Take a compass bearing of the Sun and note the exact time. The tool takes the true azimuth of the Sun for that moment from ephemerides; compass error is the true azimuth minus the compass bearing, easterly error positive. For a magnetic compass the variation is subtracted from the error, leaving the deviation on that heading.

When the azimuth changes fast: for a high body and for bodies close to the meridian. The tool calculates the rate of change of the azimuth and warns if a half-minute timing error gives more than 0.3°. In addition, the error from a tilted azimuth circle grows with the altitude of the body.

A gyro compass does not react to the ship's magnetism, and with speed and latitude set in the instrument its error is practically the same on every heading. Without those corrections a gyro compass has a speed error that depends on the course. For a magnetic compass the error is variation plus deviation, and deviation changes with heading, which is why it is tabulated.

Deviation on a heading is described by the formula δ = A + B·sin CC + C·cos CC + D·sin 2CC + E·cos 2CC. A is the constant part, B and C the semicircular deviation, D and E the quadrantal deviation. The coefficients are fitted to eight headings by least squares, and the residual error shows how consistent the observations are with each other.

Swing the ship through eight headings 45° apart and on each record one of three things: the magnetic course by comparison with another compass, the gyro course with a known gyro error, or the compass bearing of a transit whose true direction is known. Choose the method with the switch above the table.

The World Magnetic Model 2025 describes the Earth's field as a whole; NOAA puts the typical accuracy of variation at about half a degree. The model does not see local magnetic anomalies marked on charts. If the chart gives a variation for the area, turn on “Variation from chart” and enter it.

Residual deviation of the standard magnetic compass should normally not exceed 3°, and of the steering compass 5°. If it is larger, the deviation is removed with correctors rather than written into a table.

Once the page has loaded, the calculation runs in the browser: the ephemerides and the magnetic field model are already on the page. The page has to be opened while there is a connection.