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    Re: Meridional Distances
    From: Chuck Taylor
    Date: 2002 Sep 17, 19:32 -0700

    In an earlier response I mentioned an error in the formula for Meridional
    Parts as given in Bowditch 1995.  I am happy to report that it (a missing
    exponent) has been corrected in the online version at, and presumably in later printings.
    At 10:09 AM 9/18/2002 +1000,  Peter wrote:
    >My new (again, new to me) formula for rhumb line goes
    >Arctan course = DLON/DMP
    >Distance = DMD/cosCourse
    >DLON means the difference in longitude expressed in minutes of arc
    >DMP means the difference in Meridional Parts
    >DMD means the difference in Meridional Distances
    The standard formulas for Mercator Sailing, using your notation, are
    Course = arctan( DLON/DMP )
    Distance = DLAT / cos( Course)
    where DLAT means the difference in latitude in minutes of arc.
    These formulas are given in paragraph 2417 of Bowditch 1995 and paragraph
    1007 of Bowditch 1981 Volume 2.
    >there is a separate, rather more complicated, formula for when the
    >course lies close to east or west.
    The use of a separate formula for when the course lies close to east or
    west is characteristic of Mid-Latitude Sailing.  Like Michael Wescott, I
    suspect that what you are looking at is some sort of hybrid method.  Would
    you mind giving us a reference that describes the method?  I have never
    heard of a table of Meridional Distances, only Meridional Parts.
    Chuck Taylor
    Everett, WA, USA

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