>>>>General sort of question:
>>>>
>>>>I'm drawing a compass and need to determine the 'shortest' direction in which to rotate the compass (clockwise or anti-clockwise) between old and new bearings (in degrees).
>>>>In most cases it is simple but when the old and new values span the 0 degree point those solutions don't work.....
>>>>
>>>>Anyone got a foolproof algorithm (language doesn't matter - just the logic)
>>>
>>>A quick suggestion (VFP code):
>>>

SIGN(ABS(a1 - a2) - 180)

>>>
>>>-1 = counter-clockwise
>>>1 = clockwise
>>>0 = either
>>>Update: I just realized it doesn't work if angles in the 3rd and 4th quadrant. Sorry, Sat night here...
>>
>>Thanks for the try :-}
>

>FUNCTION AzimuthChange
>
>LPARAMETERS ;
>	tnAzimuth1 ;
>	, tnAzimuth2
>
>* If we're only concerned about the *change* in Azimuth,
>* we can rotate our frame of reference so tnAzimuth1 is always 0 (due north)
>* so we can avoid problems with Azimuth changes that span 0
>
>LOCAL ;
>	lnFrameRotation
>
>* Calculate how much we have to rotate the frame of reference so that m.tnAzimuth1 is at 0 degrees:
>m.lnFrameRotation = MOD( 360 - m.tnAzimuth1, 360 ) && MOD() handles case where m.tnAzimuth1 = 0
>
>* Add that amount to the second Azimuth:
>m.tnAzimuth2 = MOD( m.tnAzimuth2 + m.lnFrameRotation, 360 )
>
>RETURN IIF( m.tnAzimuth2 > 180, m.tnAzimuth2 - 360, m.tnAzimuth2 )
>

That looks good ! Thank you. All I need to know is that if the returned value is negative I need to go anti-clockwise and vice versa.