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    Re: Deviation Card with GPS
    From: Gary LaPook
    Date: 2006 Jul 29, 14:24 -0500

    From Gary LaPook:

    As long as we are discussing compass deviation and compass correction
    can anybody explain this one?

    My car has an electronic compass installed and the directions for
    correcting it for deviation (of course the car manual didn't use these
    technical terms) has you pushing a calibration button and then driving
    the car in a slow circle. How does the compass figure out the deviation
    from just the data it can capture while the car is driven in a circle?


    George Huxtable wrote:

    >Lars Bergman wrote-
    >
    >Some findings about Robert's deviation (having received his data off
    >list):
    >
    >Compass deviation could (approximately) be described by the equation
    >
    >dev = A+B*sin(cc)+C*cos(cc)+D*sin(2*cc)+E*cos(2*cc),
    >
    >where cc is the compass heading. Making a least square fit of Robert's
    >data (defining easterly deviation as positive) into this equation I
    >found the coefficients to be
    >
    >A=-1.9d
    >B=-18.5d
    >C=12.3d
    >D=1.4d
    >E=-1.5d
    >
    >Coefficient A represents a fixed offset, this could be due to e.g.
    >alignment error of lubber line or an errouneous variation value used.
    >Coefficients B and C represent permanent magnetic field components,
    >fore-and-aft and athwartship respectively. B and C are the main
    >contributors to the deviation in this case.
    >
    >The greatest deviation is found on a SE compass heading, amounting to
    >25d W. With such large values there are a few reasons to compensate
    >the
    >compass by external magnets, or finding a new location for the
    >compass,
    >or finding and removing the magnetic source creating the deviation.
    >
    >One reason is safety: If it for some reason is necessary to steer a
    >reversed course in a hurry, the easiest way is to make a 180d turn of
    >compass heading. With Robert's compass, worst case, the resulting
    >actual
    >change of ship's heading will be 43d off. This happens at cc=135 where
    >the deviation is 25W, thus magnetic=110. Turning to cc=315 the
    >deviation
    >is 18E, thus magnetic=333 instead of the desired 290d. I have never
    >seen
    >this phenomena described anywhere, and I don't think it is very well
    >known.
    >
    >Another reason is that the compass on certain headings will be "slow"
    >or
    >"fast", i.e. a certain change of actual ship's heading do not
    >correspond
    >to the change of compass heading. With Robert's compass we can look at
    >cc=205 with dev=5W making mag=200. If we make a ten degree on compass
    >turn to starboard we find the deviation of cc=215 to be 1W, thus
    >mag=214. Although the ship's head had turned 14 degs, the compass
    >shows
    >only ten. The compass is "slow". When changing course from cc=055 to
    >cc=065 we will find that the ship's actual change of heading is only 7
    >degrees; the compass is too "fast".
    >
    >=============end of Lars Bergman quote.
    >
    >Comment from George-
    >
    >As usual from him, there's good insight in Lars Bergman's analysis,
    >about Robert Eno's compass errors.
    >
    >The major problem that surfaces is that of the two major
    >contributions, B and C, which correspond to permanent magnetism,
    >perhaps originating from the engine.
    >
    >And it's the crosswise B component that's particularly worrying.
    >Presuming that the engine is mounted fore and aft, it implies a lot of
    >magnetisation, transverse to the engine block. To me, that's something
    >of a surprise. It prompts a number of questions for Robert to ask
    >himself.
    >
    >Is the compass (or perhaps even the engine) mounted a long way off the
    >line of symmetry of the boat?
    >
    >Does the compass share a panel with other instruments or components
    >close by that could be generating their own magnetic field? I'm
    >thinking particularly of a windsceen wiper motor, an inverter or
    >anything else with a transformer in it, loudhailer, analogue meters.
    >Is it mounted in a bulkhead, in which case, what's on the other side
    >of the bulkhead?
    >
    >Is the compass nearer to the engine than it really needs to be? For
    >example, can it be mounted close under the roof of the steering cabin,
    >just as high up as it can possibly go?
    >
    >Is there any vertical mast or other component close by, that's made of
    >steel?
    >
    >Robert has to be aware that he is especially vulnerable to the effects
    >of magnetised metal on board, because of his high magnetic latitude
    >and the resulting weakness of the horizontal component. And
    >particularly vulnerable to any nearby piece of vertical(ish) metal,
    >that might come to an end near to the level of the compass
    >
    >Lars puts an interesting slant in his analysis, that I haven't come
    >across before, in pointing out zones where the compass was "fast"
    >(turning faster than the boat does) and conversely slow at others. I
    >take it that Robert's course, in Frobisher Bay, when his wildly
    >varying compass showed up, was roughly Northwest. I might have
    >expected such instability to result from being in a particuarly fast
    >zone, but my understanding of Lars' analysis indicates a slow zone
    >around that direction.
    >
    >I agree completely with Lars, that this compass installation indicates
    >a particularly dangerous state of affairs. It needs some sort of
    >urgent remedy. I would think hard about installing some sort of
    >remote-reading compass up a stick, if it was my boat.
    >
    >I don't know how much change of latitude comes into Robert's voyaging,
    >but he should be aware that any deviation table or magnetic correction
    >will be local to his high latitude, and may need redoing if he
    >journeys far South (or North).
    >
    >George
    >
    >contact George Huxtable at george@huxtable.u-net.com
    >or at +44 1865 820222 (from UK, 01865 820222)
    >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >>
    >
    >

    >


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