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Subject: Re: Deviation plot
From: Lars Tornqvist (lpt@XXX.XXX)
Date: Wed Feb 06 2002 - 07:13:23 EST
Hi
If anyone is interested I've got an Excel spreadsheet which draw your
Deviation courve and calculate the Coefficients.
Let me know then I will download it on my home page.
Lars
----- Original Message -----
From: Trevor Kenchington <Gadus@XXX.XXX>
To: <NAVIGATION-L@XXX.XXX>
Sent: Tuesday, February 05, 2002 10:48 PM
Subject: Re: Deviation plot
> John Kabel wrote:
>
> > 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!!
>
>
> I think the much simpler and more general solution was posted a few days
> ago but I did not keep the message and so cannot say who provided it. To
summarize:
>
> 1) Put your boat on any course you wish, with any amount of leeway and
> any current running -- but with reasonably flat water so that you can
> get accurate measurements. Hold its heading steady until the compass
settles.
>
> 2) Select a visible, charted object at a moderate distance. Determine
> its position using a WGS84 chart (or with appropriate corrections from
> any other chart).
>
> 3) Enter that position into your GPS as a waypoint and have the GPS read
> out the true bearing of the object/waypoint.
>
> 4) Record, simultaneously, the GPS bearing, the compass heading of your
> boat (using the steering compass) and the compass bearing of the object
> (also using the steering compass). The latter can be obtained directly
> if the steering compass has sighting vanes. If not, use a pelorus or a
> hand-bearing compass (which will need dual readings for heading and
> bearing) to get the relative bearing and thence calculate the compass
> bearing by the steering compass.
>
> 5) The difference between the GPS's true bearing and the compass bearing
> is the compass error. Deducting the variation leaves the deviation of
> your steering compass for the heading at the time the three angles were
recorded.
>
> 6) Repeat for other headings.
>
>
> In the absence of a GPS, the same thing can be done using two charted
> objects to establish an LOP of known bearing and then recording heading
> and the compass bearing to the two objects as you cross the LOP.
>
>
> Trevor Kenchington
>
>
> --
> Trevor J. Kenchington PhD Gadus@XXX.XXX
> Gadus Associates, Office(902) 889-9250
> R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251
> Nova Scotia B0J 2L0, CANADA Home (902) 889-3555
>
> Science Serving the Fisheries
> http://home.istar.ca/~gadus
>
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