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> -----Original Message-----
> From: Navigation Mailing List
> [mailto:NAVIGATION-L@XXX.XXX]On Behalf Of Bob Hendry
> Sent: Monday, January 31, 2000 12:28 PM
> To: NAVIGATION-L@XXX.XXX
> 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
ayscue@XXX.XXX

   

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