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    Re: going overboard on decimals
    From: Millard Kirk
    Date: 1999 Jul 24, 7:30 PM

            For the last two summers I have sailed on a boat called Indian Summer where
    the captain plots the courses on his charts and converts each leg to
    magnetic for the compass course to steer.  He then enter the leg  into his
    GPS, and if the course to steer of the GPS does not correspond to the course
    laid out on his charts he would then begin to resolve the difference.  We
    would sail the leg using our Compass Course from the charts after resolving
    any differences.    The GPS was usually the same course we had plotted or
    the difference was so slight it was not worth worrying about. Most of the
    time the slight difference was in the variance figured from the charts.
            It was interesting to watch the CMG shown by the GPS differ from my DR.  On
    short legs it was insignificant compared to the DR, but on the longer legs
    there was some difference on I would like to get some input.  After the GPS
    showed a bearing of 1 degree difference from our course I was wondering how
    much off course that represented, without plotting of course.   He made a
    statement that we were only 1 nautical mile off course.  I was able to
    understand from his discussion that may be true  if one is at least 60 miles
    from his destination one (1) degree would calculate about that.  I have
    since run across a rule "The One In Sixty Rule" although not fully
    explained, but I think it is what he was referring to.
            In any case after about two days we spotted the Block Island we were off
    course about  1.5 miles. We did not miss the island, but needed  to look at
    the charts again to orient ourselves as to where we wanted to be and avoid
    any hazards.
            Oh by the way on this adventure his Main GPS failed and so did his backup
    GPS.  It was nice to have the DR and the notes as to when we passed certain
    land marks.  At the first opportunity he purchase another GPS.
    Let's assume a day's run is 240 miles, more or less.  If average speed
    is off by 1%, that's 2.4 miles per day.  If course made good is off by
    1 degree, that's 4.2 miles per day.  Imagine how much worse this gets
    when you're being beaten up by a storm.
    I reported my results to only 1', and even that was overkill.
    Learning the Hard Way!!
    Millard Kirk KB8YQO       | mailto:mkirk@marshall.edu
    116 Lewis Ave                    | http://webpages.marshall.edu/~mkirk/
    Barboursville, WV               | A West Virginia Blue Water Sailor
    25504                                 | Phone: (304) 736-6544
                                               | First United Methodist Church,
    Barboursville, WV
                                               | Homepage

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