On 08/24/2008 01:53 PM, James Turner wrote: > Doing this, I came across something which seems counter-intuitive to me:
I agree, it's counterintuitive, to say the least. > default azimuth to offset by is the *reciprocal* runway heading. This > means to start 10nm 'out' from the threshold, one can use a positive > value of 10 - I expected this to require a negative value. Good catch. I agree, there are many good reasons why a negative value would be appropriate here. > I wonder if this is intentional, since it's the more common case that > starting 'ahead' of the threshold, but equally it might be a bug - the > azimuth is inverted to move from the runway centre 'back' to the > threshold. Long-ago discussions indicate that it is intentional. But it is still a Bad Idea. The problem is, starting "ahead" of the threshold is only one use-case among many. It is not even remotely the most common case chez moi. It is a Bad Idea to simplify this one case at the cost of logic and consistency among the many other cases, such as positioning relative to enroute waypoints. On a tangentially related note, it appears that James Turner is wise enough to live by the dictum, "principles and concepts first; terminology second". I mention this because previous discussions of this topic quickly degenerated into pedantic emphasis on the definition of "distance" (which is necessarily positive) to the exclusion of more usable concepts such as location-vectors and position-vectors along the number line. For years, the _Sport Model_ has represented offsets in the logical way, such that negative offsets are on the "upstream" side of the reference point, while positive offsets are on the "downstream". side. > Anyway, what makes me wonder if there's a bug here is the glideslope > logic. In the case where a glideslope angle is specified, and also a > preset altitude, we encounter the code at line 976 of fg_init.cxx. > Now, the crucial observation is that this code does multiply the final > distance by -1. As a result, the calculated offset-distance-nm would > place the start position well down the runway - possibly some way > beyond it, in fact. ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
