On Friday 14 May 2004 22:13, Andy Ross wrote: > Durk Talsma wrote: > > I want to calculate the current position of the aircraft , sssuming > > that it travels along a great circle route between these two points > > and also assuming that the current time is somewhere between arrival > > and departure. > > Probably the simplest approximation is to convert both locations to > cartesian coordinates, interpolate a path between them, back-convert > the interpolated to a lat/lon pair. The altitude will be wrong, of > course, but you probably don't care. Remember to do the interpolation > in angle-space: make a triangle with the origin and the two points and > sweep a vector across the triangle by angle; don't just linearly > interpolalte along the line. > > That approximation assumes a spherical earth (for non-spheres, the > constant-angular-speed interpolation isn't quite right). If you > really want to use a ellipsoid earth model, then the problem becomes > hard to solve analytically. One way to handle this would be to chop > up the interpolated path into a large number of smaller segments, back > convert them to lat/lon/alt as above, *re-convert* to cartesian values > to compute a distance table, and then linearly interpolate them > between the points. > > Andy >
Andy, That's cool, the interpolation procedure you're suggesting is conceptually very similar to the solution I had in mind, the only difference being that I was going to try and do the whole solutions using polar coordinates. Using cartesian coordinates probably simplifies the procedure quite a bit. After sending my mail, I realized that doing these calculations in WGS84 coordinates is probably not even necessary: all I want to do is get a rough approximation of the position of an aircraft, and the assumption that is flies a perfect great circle nav route, instead of a real world flight plan already introduces an estimation error that is probably far greater than the one introduced by assuming a sperical model. I was playing a lot with WGS84 code yesterday, so that was what my state of mind was in, I guess. Thanks, Durk Thanks, Durk _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
