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    Re: Deviation plot
    From: Lars Tornqvist
    Date: 2002 Feb 6, 12:13 -0000

    Hi
    
    If anyone is interested I've got an Excel spreadsheet which draw your
    Deviation courve and calculate the Coefficients.
    Let me know then I will download it on my home page.
    
    Lars
    
    ----- Original Message -----
    From: Trevor Kenchington 
    To: 
    Sent: Tuesday, February 05, 2002 10:48 PM
    Subject: Re: Deviation plot
    
    
    > 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{at}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
    >                      http://home.istar.ca/~gadus
    >
    
    
    

       
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