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    Re: Deviation plot
    From: Chuck Taylor
    Date: 2002 Feb 4, 12:47 US/PACIFIC

    The documentation for my Garmin Etrex Mariner GPS defines the following data
    Bearing - The direction from your current location to a destination.
    Course  - The direction from your starting location to a destination.
    Heading - Your moving direction.
    With my Garmin, you can select which of these values you wish to view on the
    various display screens.
    Heading does involve averaging, I believe.
    In a cross current, getting Course and Bearing mixed up can be a real problem.
    If you blindly follow Bearing instead of Course, you could be swept wide of your
    intended track.
    So long as the waypoint is several miles distant, the Bearing should not change
    enough to matter with the minimal motion required to turn the vessel even if
    there is averaging involved.
    Terminology may differ from manufacturer to manufacturer.  One should check
    his/her own manual to be sure.  Garmin GPS manuals are available for download in
    pdf format from the Garmin website.
    > Are you sure this works?  Track and heading are averaged figures.  Is bearing
    instantaneous figure?  I had guessed there was a bit of averaging going on with
    it to take
    out anomolies in signals received.  This particular one might be too fast to
    > Original Message:
    > -----------------
    > From: Chuck Taylor ctaylor@PREMIER1.NET
    > Date: Mon, 4 Feb 2002 08:48:04 US/PACIFIC
    > Subject: Re: [NAV-L] Deviation plot
    > > > Are there any objections to use a GPS to make a deviation
    > > > plot for my compass?
    > >
    > > GPS tells you which way you are going not necessarily which
    > > way you are pointed. They aren't always the same.
    > Yes, but you can easily use a GPS to check your compass as follows:
    > 1. Select a visible landmark or fixed navigational aid at least several miles
    > distant.
    > 2. From the chart, set the coordinates of that point as a waypoint in your
    > 3. Set your GPS to navigate to that waypoint.  It will give you the true
    > from your present position to that waypoint. (Be sure to read "Bearing", not
    > "Heading" from your GPS.)
    > 4. Remaining in more or less the same position, turn your vessel in a slow
    > circle, recording both ship's heading (per ship's compass) and the compass
    > bearing of the waypoint for every 15 degrees or so of heading.
    > 5. Use the data gathered in step 4 along with the bearing from the GPS and the
    > variation from the chart to compute a deviation table.  (Don't forget to apply
    > the annual change in variation listed on the chart.)
    > Another point is to make sure the horizontal datum on the chart you are using
    > matches the horizontal datum of the GPS (e.g. WGS 84).
    > Chuck Taylor
    >  47 degrees 55.161 minutes North Latitude
    > 122 degrees 11.176 minutes West Longitude
    > --------------------------------------------------------------------
    > mail2web - Check your email from the web at
    > http://mail2web.com/ .

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