NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Shaking off the rust
From: Steve Ayscue
Date: 2000 Mar 02, 14:16 EST
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 > 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