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    Re: Shaking off the rust
    From: Steve Ayscue
    Date: 2000 Mar 02, 14:16 EST

    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L@XXX.XXX]On Behalf Of Bob Hendry
    > Sent: Monday, January 31, 2000 12:28 PM
    > Subject: Re: Shaking off the rust
    > Hello Derrick
    > On 31-Jan-00, you wrote:
    > > Rick,
    > >
    > > The problem is that you use DR for planning purposes. By definition, DR
    > > plots do not take into account variation, currents, winds,
    > sloppy steering
    > > or any of the other items that impact the course that is steered. In
    > > addition, according to Bowditch and others, you should plot
    > everything in
    > > true on the chart. So, you don't plot the course taking into account the
    > > variation (local/regional magnetic differences between
    > true/magnet north)
    > > or deviation (magnetic changes based on your vessel).
    > >
    > > Now, you can take each of these elements into account and have a revised
    > > planned course on your chart. But that is not a DR plot. When you steer
    > > the revised plot, you will still have to take into account the variation
    > > and deviation, current, winds, sloppy steering and the other items that
    > > impact the course actually.
    > >
    > >        derrick
    > Thanks everyone for your advice - I think the cobwebs have
    > started to clear
    > away but, there is a hint left so let's state the problem in another way.
    > (1) - My chart shows "Variation 22 degrees 45 minutes W (1992)
    > decreasing 2
    > minutes annually.
    > (2) - I have a Deviation Table compiled for my boat.
    > (3) - I can determine the Lat and Long of a number of waypoints from the
    > chart.
    > (4) - TVMDC will be used to plot the course from one waypoint to the next.
    > (5) - What do I enter as a waypoint position into the GPS? Do I
    > use the True
    > position as taken off the chart?
    > StillRustyBob
    You would use the position gathered from the Lat/Long lines from the chart
    without regard to any variation from the chart (or deviation from your
    compass). However, you *must* be sure to use the same datum settings on your
    GPS as the one your chart is based on. If you are using bearings (and
    distance) from a known point on the chart to calculate a new position in the
    GPS, then yes, you would need to apply the variation factor to the bearing.
    If you were using a bearing and distance from your vessel to calculate a
    waypoint in your GPS, then you would need to apply the deviation factor into
    your bearing.
    Steve Ayscue

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