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Deviation, Variation and Heading Offset
12-21-17, 06:26 AM (This post was last modified: 04-04-18 02:40 PM by Chuck - Raymarine - Moderator.)
Post: #1
Deviation, Variation and Heading Offset
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.

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07-07-19, 06:26 AM (This post was last modified: 07-09-19 10:11 AM by Derek - Raymarine - Moderator.)
Post: #2
RE: Deviation, Variation and Heading Offset
I have the 2004 set of Raymarine autopilot instruments for my Beneteau 423 sailboat with the autopilot head replaced with a p70. When I do the circles for calibration, everything is fine for a day or so but then the heading flips 180 degrees. It doesn’t seem to affect performance of the autopilot and I’ve lived with it for years.

However, I’ve recently installed a Vesper XB-8000 AIS transreceiver and it uses the heading to set the boat icon orientation. When displayed on the AIS of other boats, I appear to be going backwards.

Is there any way to just manually adjust the heading to any value I want, even 180 degrees off?

Autohelm 6000
p70 head
ST60 compass
Chartplotter RI70CRCplus

Thanks, Bob423
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07-09-19, 10:15 AM
Post: #3
RE: Deviation, Variation and Heading Offset
Hi Bob,

The issue is likely that the p70 will not calibrate the older generations of Autopilot. the p70 will control them AUTO, STANDBY etc . . . but you cannot access the compass cal on the earlier computers.

If you could borrow an ST6000/ St6001 or ST6002, connect via SeaTalk, then you could change the compass heading in the calibration by up to 180 degrees.

On another point, if the computer repeatedly 'forgets' the compass heading offset, then it sounds like the Eeprom in the computer is failing and the computer might need to be replaced as spares of the older computers are few and far between.

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07-10-19, 05:52 AM (This post was last modified: 07-11-19 10:53 AM by Derek - Raymarine - Moderator.)
Post: #4
RE: Deviation, Variation and Heading Offset
Thanks for the quick reply! I had the issue before I replace the old head which was failing with the new p70. I was hoping the problem was with the head and the p70 would correct the problem. As of yet, I haven't done the 720° circle to calibrate the autopilot with the p70 head.

I was looking for a way to enter the 180° offset directly but couldn’t find the screen on the p70 to use. I’m thinking I need to enter dealer calibration mode but I don’t see the instructions on how to do that in the users manual. Can you tell me how?

If the eprom is bad, then I’m out of luck, I guess, unless I could replace only the eprom. Is that possible?
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07-11-19, 10:57 AM
Post: #5
RE: Deviation, Variation and Heading Offset
Hi bob423

Unfortunately the only way to adjust the heading offset is through an older SeaTalk control unit, there is no way to do it via the p70.

Unless you can borrow an older head to change the offset - which is stored in the computer, then the computer will need to be updated.

One point to note is that the autopilot doesn't care what the heading is - all it wants to do is steer straight - the heading becomes important if tou are using the pilot with an MFD and then heading is needed to orientate the boat icon and when engaging track mode.

The Eeprom is soldered on to the PCB and so replacement is an unlikely possibility.

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