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Subject: Re: Deviation plot
From: John Kabel (jjkabel@XXX.XXX)
Date: Tue Feb 05 2002 - 12:31:43 EST
Several guys mashed keys:
> At 08:15 AM 2/5/02 +0000, Chuck Taylor wrote:
> >> on 2/4/02 12:47 PM, Chuck Taylor at ctaylor@XXX.XXX wrote:
> >>
> >> > The documentation for my Garmin Etrex Mariner GPS defines the following
> >data
> >> > items:
> >..
> >> > Heading - Your moving direction.
> >
> >And John LeRoy replied:
> >
> >> Unfortunately the guy who wrote the manual is wrong.
> >>
John K. replies:
The point of the discussion seems to be using GPS to determine deviation.
GPS cannot determine HEADING of a vessel. The receiver measures the
direction, velocity and position of the antenna of the receiver, essentially. It
has no way of knowing which way the bow of the boat is pointed.
The best you can do is carefully navigate toward a known landmark a long
distance off (to help average out swings of the bow) and take a number of
readings of COMPASS heading (compass fixed on the boat) while at the
same time recording TRUE heading (direction the GPS receiver is moving).
Assuming you don't move about on the boat, and take a reasonable number
of measurements to establish a meaningful average for the headings for each
landmark, solving for deviation will be easy. What's reasonable? I would do
eight or ten for each heading, and this would require assistance and some
time. Just how bad do you think your compass deviation is? If bad, spend
lots of time and do it right!!
Doing this for six or eight landmarks spaced around the compass will get you
a fairly accurate deviation chart. Graphing will give a smooth curve, better
than a table.
Do this on reasonably smooth water, so waves don't knock the boat heading
about too much. It doesn't work on my Sea-Doo, big as it is.
Note that this is possible only on powered craft. Anything with a sail relies on
a keel and has leeway. I've tried, and I do not believe it to be possible to
remove or solve for leeway. I would love to hear from someone who has been
able to do that!!
John Kabel
London, Ontario
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