Hi, I have a problem with the following: I made a proggy, which will use GC and partially Rhumb Line formulas to calculate the lat/lon of a point D, which is different nms out (with a given heading) of a starting point with a given lat/lon/heading. Now all this working quite well. It works great for small distances. So far, the farer the distance, the more "off course" the calculated point is. Let's say the starting point is the starting point of the centre of a runway. Now I want a perfectly 180° turned around line to a point D, which is let's say 5nm out of the starting point. No matter if I use GC or rhumb line, my point D is always a little bit off course (meaning a little bit right of a thought perfect line). Interestingly, if I do the calculations in Europe, the mistake is not as big as in US. Magnetic variation should not matter I think, since FS should calculate everything with true north-based headings, so i think it is not this (I also tried so far, did not help me much). I would be very thankful about: 1) An idea, what might cause the off-course 2) A hint, what to do against (depends on 1 most probably) 3) Perhaps I should neither use GC nor Rhumb lines, but a - for me still secret - other formula? Any comments are really very welcome! Thanx alot, Chris.