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Subject: Re: Bowditch
From: Casey. Noel (Noel.Casey@XXX.XXX)
Date: Mon Jan 29 2001 - 07:58:30 EST
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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