[Rhodes22-list] Magnetic or True? By a famous nautical author... Other th...

[bill davidge]

Dave
    What do you know or think about the Garmin  398? I'm just learning about Navigating in a Cost Guard  class and have a long way to go before I know which way to add or subtract the deg. to the Compass from the compass rose on the chart 
 
   Thanks Bill Davidge Temperance MI.

    

----- Original Message ----
From: "DCLewis1 at aol.com" <DCLewis1 at aol.com>
To: rhodes22-list at rhodes22.org
Sent: Tuesday, April 10, 2007 12:06:30 AM
Subject: Re: [Rhodes22-list] Magnetic or True? By a famous nautical author... Other th...


Bill and Ed,

I don't think it matters much if I were to try to sail a true heading as  
magnetic, my chart plotting GPS  would straighten me out  quickly.  With a chart 
plotting GPS I enter  my destination/way points on the electronic charts in 
the GPS unit and  then  I  watch my track evolve in real time as I sail.  Of  
course, I always try to start on the right magnetic bearing, since I steer via  
compass, but it's not critical at all.  If I'm not headed where I want to  be 
headed relative to my desired way point, the plot on my chart plotting GPS  
will tell me immediately and quantitatively.   I can look at the  track display 
that's automatically displayed (no transferring plot points)  and instantly 
know if my course is too high or low and with that crude info  I can make a 
steering adjustment, note the new reading on the compass,  and then hold that new 
compass reading.  I  never make the  calculation to go from degT to degM, 
instead I just note I'm off  track and adapt.  

As to deviation and variance, I'm not sure it matters when   a compass is 
used with a chart plotting GPS.  The compass becomes a  relative instrument that 
is used to hold a heading appropriate to keep on,  or near,  the desired trip 
track, that's it.  The numbers on the dial  are just a way to help you keep a 
heading - whatever that heading  really is, and that heading is ultimately 
determined by your chart plotting  GPS.  Making progress to the next way point is 
what's important, and you  can see that evolve (or not evolve) on the GPS 
display.  Steering  accurately by compass, as per John Vigor, is now besides the 
point - I use  the compass to hold a bearing that will keep me on my desired 
track (or get me  back to my desired track) and that track is monitored 
accurately in real time by  my chart plotting GPS.

And all of the above is accomplished without keeping my head down  navigating 
via GPS because the chart plotting GPS unit does all of the  calculations and 
plotting.  All I have to do is glance at the display  to see that I'm on 
track or off track and if I'm off track what I have to do to  get back on track.  
If I want to know quantitatively how far off-track I  am, speed, or whatever, 
I have to change the display page - but that's pretty  quick & simple.

And with GPS I always know where I am (Lat/Lon) much better than the  
paper-plotters.  I don't think there's any question about that.

Of course we should always know how to fall back to paper and pencil,  but I 
think that fall back is a long ways back.  I think a chart plotting  GPS can 
really be a very useful asset - and it can be an especially useful  asset at 
night or in inclement weather (low vs).

Dave



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