Visual Basic's Atn function only returns values between Pi and -Pi. This routine shows how to find correct angles in the other two quadrants.

You pass the Atan2 function the X and Y offsets in the direction of interest. The function uses the signs of the offsets to figure out which quadrant to return. It looks for very small X offsets which would produce an undefined error in calculating Atn(y / x) and uses Atn to calculate other values.

Public Function Atan2(ByVal y As Double, ByVal x As Double) _
    As Double
  Dim theta As Double

  If (Abs(x) < 0.0000001) Then
    If (Abs(y) < 0.0000001) Then
      theta = 0#
    ElseIf (y > 0#) Then
      theta = 1.5707963267949
    Else
      theta = -1.5707963267949
    End If
  Else
    theta = Atn(y / x)
  
    If (x < 0) Then
      If (y >= 0#) Then
        theta = 3.14159265358979 + theta
      Else
        theta = theta - 3.14159265358979
      End If
    End If
  End If
    
  Atan2 = theta
End Function