[LINK] F-22 Raptor stumped by International Date Line

Gordon Keith gordonkeith at acslink.net.au
Fri Mar 2 08:34:22 AEDT 2007

On Fri, 2 Mar 2007 00:54, Adam Todd wrote:
> Gordon, I like the idea of the inverting 180, but really, you don't think
> that the Americans would design a $125 million plane and limit it's use
> only to the Norther Hemispeher?

I was referring to the to the change from western to eastern hemisphere.

I work with ship based data and I'm very aware of the problems associated with 
the calculations that cross the date line (actually the hemisphere boundary), 
but for the work I do it's often too much trouble to properly code.  One day 
it may well be a problem, but the cost/value isn't there. I don't work with 
critical software.

As you travel west your longitude is decreasing, until you get to 180 degrees 
west when suddenly you are at 180 degress east. If you start at 150 degrees 
west and travel 40 degrees west you are now 320 degrees east of where you 
started. (-150 - 40 = 170)

Likewise if you start at 150 east  (Sydney) and travel 55 degrees east you 
wind up at 155 west (Hawaii), 150 + 55 = -155.

If your navigation system knows you are supposed to go west and it suddenly 
discovers you are a long way east of where you were, it could get confused. 

The special case of navigation crossing hemispheres is one that has to be 
dealt with, but I suspect not many programmers think of the date line as a 
theatre of operations.

Any test in any location that didn't include actually crossing the line would 
not have a problem.


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