You almost certainly don't need this logic: 

if y > 0 then
     if x > 0 then tc1 = arctan(y/x)
     if x < 0 then tc1 = 180 - arctan(-y/x)
     if x = 0 then tc1 = 90
   if y < 0 then
     if x > 0 then tc1 = -arctan(-y/x)
     if x < 0 then tc1 = arctan(y/x)-180
     if x = 0 then tc1 = 270
   if y = 0 then
     if x > 0 then tc1 = 0
     if x < 0 then tc1 = 180
     if x = 0 then [the 2 points are the same]

[download]

Just feed y and x to atan2 and it will take care of the quadrant logic.

Eg. This prints the angle in radians for the four cardinal directions followed by those for the four intercardinal directions:

print atan2( $_->[0], $_->[1] ) 
    for [ 0,-1],[ 0,+1],[-1, 0],[+1, 0],
        [+1,+1],[-1,+1],[-1,-1],[+1,-1];;
3.14159265358979
0
-1.5707963267949
1.5707963267949
0.785398163397448
-0.785398163397448
-2.35619449019234
2.35619449019234

[download]
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In reply to Re: Calculate aircraft headings from GPS data for google kml display by BrowserUk
in thread Calculate aircraft headings from GPS data for google kml display by hujunsimon

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