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    Re: Deviation plot
    From: Trevor Kenchington
    Date: 2002 Feb 5, 18:48 -0400

    John Kabel wrote:
    > The best you can do is carefully navigate toward a known landmark a long
    > distance off (to help average out swings of the bow) and take a number of
    > readings of COMPASS heading (compass fixed on the boat) while at the
    > same time recording TRUE heading (direction the GPS receiver is moving).
    > Assuming you don't move about on the boat, and take a reasonable number
    > of measurements to establish a meaningful average for the headings for each
    > landmark, solving for deviation will be easy.  What's reasonable?  I would do
    > eight or ten for each heading, and this would require assistance and some
    > time.  Just how bad do you think your compass deviation is?  If  bad, spend
    > lots of time and do it right!!
    > Doing this for six or eight landmarks spaced around the compass will get you
    > a fairly accurate deviation chart.  Graphing will give a smooth curve, better
    > than a table.
    > Do this on reasonably smooth water, so waves don't knock the boat heading
    > about too much.  It doesn't work on my Sea-Doo, big as it is.
    > Note that this is possible only on powered craft.  Anything with a sail relies on
    > a keel and has leeway.  I've tried, and I do not believe it to be possible to
    > remove or solve for  leeway.  I would love to hear from someone who has been
    > able to do that!!
    I think the much simpler and more general solution was posted a few days
    ago but I did not keep the message and so cannot say who provided it. To summarize:
    1) Put your boat on any course you wish, with any amount of leeway and
    any current running -- but with reasonably flat water so that you can
    get accurate measurements. Hold its heading steady until the compass settles.
    2) Select a visible, charted object at a moderate distance. Determine
    its position using a WGS84 chart (or with appropriate corrections from
    any other chart).
    3) Enter that position into your GPS as a waypoint and have the GPS read
    out the true bearing of the object/waypoint.
    4) Record, simultaneously, the GPS bearing, the compass heading of your
    boat (using the steering compass) and the compass bearing of the object
    (also using the steering compass). The latter can be obtained directly
    if the steering compass has sighting vanes. If not, use a pelorus or a
    hand-bearing compass (which will need dual readings for heading and
    bearing) to get the relative bearing and thence calculate the compass
    bearing by the steering compass.
    5) The difference between the GPS's true bearing and the compass bearing
    is the compass error. Deducting the variation leaves the deviation of
    your steering compass for the heading at the time the three angles were recorded.
    6) Repeat for other headings.
    In the absence of a GPS, the same thing can be done using two charted
    objects to establish an LOP of known bearing and then recording heading
    and the compass bearing to the two objects as you cross the LOP.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                        Science Serving the Fisheries

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