Deviation, Variation and Heading Offset

[Derek - Raymarine - Moderator]

All compasses, whether Electronic or Fluid filled - 'Wet-card' type will be subject to both Deviation and variation which will affect the accuracy and the performance of the compass.

Deviation is caused by local magnetic distortion of the earth’s magnetic field by ferrous materials, electric/ electronic devices, mobile phones, loudspeakers etc . . mounted nearby. The Effect of deviation can be reduced by correct location of the compass sensor, click here and and click here for more information regarding compass location and calibration.

The deviation figure displayed by the Autopilot is the measured value of the maximum error plus the minimum error, between the expected compass heading and the measured compass heading, when the compass sensor is calibrated (Linearised). Correction to compensate for this distortion is applied automatically to each heading after the linearisation process is completed, to provide an accurate magnetic heading display.

The electronic compasses, whether the earlier style Fluxgate sensor or the current EV1 or EV2 sensor have the capability to compensate (Automatically Linearise) for local Deviation, whereas typical fluid filled or 'wet card' compasses have to be manually adjusted with magnets by a qualified compass adjuster. You can click here for more information

Variation is the actual local difference between Magnetic North and True North, this is a global magnetic anomaly, the value of variation changes depending on geographical location and date, since the location of Magnetic North changes with time. Variation cannot be compensated for mechanically or electronically, but is a value which is used by navigators to correct locally for True heading based navigation or plotting magnetic based courses onto paper charts which are generally orientated to True North.

Heading offset is a value which is applied to the displayed heading of an electronic compass in order to accurately align it with the correct magnetic bearing.
Typically, once an older electronic fluxgate compass has been linearised, there might be a small but constant positive or negative error on every heading once the deviation has been identified and applied.

The heading offset enables the user to effectively ‘rotate’ the compass through a uniform angle clockwise or anticlockwise to eliminate this constant error and enable corrected bearings to be displayed.

Later electronic compass sensors have built-in heading offset correction and so this manual adjustment is rarely needed.

RAYFAQ#