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    Re: Bowditch
    From: Roger M. Derby
    Date: 2001 Jan 29, 18:07 EST

    It was quite doable.  It's just that I was born of Friday the 13th and
    computers are inherently evil.  They will find the wrong answer if there is
    any ambiguity available.  (Try testing a floating point number for zero as a
    loop terminator.  I had to test for less than 10E-32 since the series never
    quite converged due to truncation errors.)
    ----- Original Message -----
    From: "Casey. Noel" <Noel.Casey@XXX.XXX>
    Sent: Monday, January 29, 2001 7:58 AM
    Subject: Re: [NAV-L] Bowditch
    > This is a well known limitation of the arctangent(x,y) function as being
    > ambiguous  for angles greater than 90 deg.
    > The function Arctangent2(x,y) resolves the issue. I don't know if the VAX
    > compiler handled that.
    > In excel spreadsheet there is the atan and atan2 functions, with the atan2
    > function being preferable in all cases.
    > > -----Original Message-----
    > > From: Roger M. Derby [SMTP:derbyrm@XXX.XXX]
    > > Sent: Friday, January 26, 2001 7:45 PM
    > > Subject:      Re: Bowditch [was Re: Hi (WAS: RE: ping)]
    > >
    > > Just a word of caution that I'm sure is unnecessary.  When I was
    > > programming
    > > a VAX to do great-circle calculations (path length and departure) using
    > > the
    > > equations from Bowditch, I got some very strange results.  These turned
    > > out
    > > to be caused by the machine's math library making different assumptions
    > > about quadrant than was used by Bowditch.  I finally made up a table of
    > > entries for two points a few nm apart using paths that hit all four
    > > quadrents and the four cardinal points and then checking against
    > > "reasonable" answers.  Had to add several IF-THENs before Singapore was
    > > long way from the Lizard and in the correct direction.
    > >
    > > Roger
    > >
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