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Subject: Re: Bowditch
From: Roger M. Derby (derbyrm@XXX.XXX)
Date: Mon Jan 29 2001 - 18:07:17 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.)
Roger
----- Original Message -----
From: "Casey. Noel" <Noel.Casey@XXX.XXX>
To: <NAVIGATION-L@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
> > To: NAVIGATION-L@XXX.XXX
> > 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
a
> > long way from the Lizard and in the correct direction.
> >
> > Roger
> >
>
>
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