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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@SEIDATA.COM]
> Sent: Friday, January 26, 2001 7:45 PM
> To:   NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> 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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