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Are you sure this works?  Track and heading are averaged figures.  Is bearing 
an instantaneous figure?  I had guessed there was a bit of averaging going on 
with it to take out anomolies in signals received.  This particular one might 
be too fast to matter though.

Original Message:
-----------------
From: Chuck Taylor ctaylor@PREMIER1.NET
Date: Mon, 4 Feb 2002 08:48:04 US/PACIFIC
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: [NAV-L] Deviation plot


> > Are there any objections to use a GPS to make a deviation
> > plot for my compass?
>
> GPS tells you which way you are going not necessarily which
> way you are pointed. They aren't always the same.

Yes, but you can easily use a GPS to check your compass as follows:

1. Select a visible landmark or fixed navigational aid at least several miles
distant.

2. From the chart, set the coordinates of that point as a waypoint in your GPS.

3. Set your GPS to navigate to that waypoint.  It will give you the true bearing
from your present position to that waypoint. (Be sure to read "Bearing", not
"Heading" from your GPS.)

4. Remaining in more or less the same position, turn your vessel in a slow
circle, recording both ship's heading (per ship's compass) and the compass
bearing of the waypoint for every 15 degrees or so of heading.

5. Use the data gathered in step 4 along with the bearing from the GPS and the
variation from the chart to compute a deviation table.  (Don't forget to apply
the annual change in variation listed on the chart.)

Another point is to make sure the horizontal datum on the chart you are using
matches the horizontal datum of the GPS (e.g. WGS 84).

Chuck Taylor

 47 degrees 55.161 minutes North Latitude
122 degrees 11.176 minutes West Longitude

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