It usually works like this: At the beginning of the flight and your IGC file you have a flat altitude line on the ground and since the ground is usually a known altitude, the further baro altitudes are compensated so that at the ground baro alt = 0. At least that's usually the way Strepla displays my flights. Obviously when you have a splitted IGC file and the second part begins in the air this calculation doesn't work and Strepla still assumes that you started on the ground..
Turbo Am 30.09.2010 11:36, schrieb Henrik Bieler: > Hey Ian, > > I am not really an expert in this field but I disagree. Provided there > are enough satellites in view I would prefer GPS Altitude for final > glide calculations over QNH corrected Pressure Alt. > If I recall some of my Flight Mechanics and Aerodynamics classes > correctly, the glide ratio L/D is completely independent of Air density, > temerature, etc. Its a function of the Wing-Profile (in our case the > whole Glider) dependent of the AOA. > Therefore, lets say for example at best L/D, with a given Altitude the > glider can travel a given distance. If you know the Altitude precisely, > you can calculate the distance. You can also think of it in terms of > pontential energy... > The GPS-Altitude is pretty precise in my opinion. The pressure altitude > (corrected by QNH) is only precise at the ground of the airport, where > the QNH was measured. > If you fly cross country you get an error, but I think more important > you get a temperature error if you fly and the atmosphere is not at ISA > condition (which it never is). > Lets say for example you do a wave flight in the spring. the temperature > is 15° less than ISA and your flying at 2500m MSL true (!) altitude. > Your QNH corrected Pressure Altimeter will show approx. 2650 m ( 4% per > 10K ISA Difference) > In Summer with Temperature at 15° above ISA, you're still at 2500m but > your Altimeter will show 2350m. > Remember the old saying: "In Winter the mountains are higher" ;-) > With GPS-Altitude, you don't get this error. > > However for vertical airspace distances, I think QNH-Alt should be the > Master, since Airspaces are defined that way. The "true" vertical > boundarys just "fluctuate" a little bit with temperature. > > Anybody know how the usual competion scoring software calculates if you > vertically violated an Airspace? The same algorythm in XCS would be the > right thing to have ;-) > > Since I don't know the XCS internal computations it would be nice to get > Max or Tobias comment on these issues. Thanks :-) > > My personal preference for the use of Altitude sources in XCS would be: > > Final glide calculations: GPS-Alt primary, fallback to QNH-Alt. > Airspace vertical distance warnings: primary QNH-Alt, secondary GPS > > Just my 2 cents... > > Henrik > > > Am 28.09.2010 10:02, schrieb Ian: >> I would think for final glide calcs, the best source of altitude data is >> (in order of accuracy): >> >> - Barometric altitude with QNH correction. (Can this be done >> automatically before take off?). >> >> - GPS altitude. >> >> - Barometric altitude without QNH correction. (Often out by 100's of feet). >> >> - Barometric "flight levels" rounded off to 100' as per the B500. >> >> I know the B500 uses barometric pressure and outside air temperature to >> provide corrections to TAS and vario readings. The "flight levels" data >> would be good enough for this purpose. I am not sure which altitude >> source it uses for its internal final glide calcs. >> >> I will have to find something else to display in that box with the "---" >> indication ;-) >> >> Thanks >> >> Ian >> >> ------------------------------------------------------------------------------ >> Start uncovering the many advantages of virtual appliances >> and start using them to simplify application deployment and >> accelerate your shift to cloud computing. >> http://p.sf.net/sfu/novell-sfdev2dev >> _______________________________________________ >> Xcsoar-user mailing list >> Xcsoar-user@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/xcsoar-user >> > > ------------------------------------------------------------------------------ > Start uncovering the many advantages of virtual appliances > and start using them to simplify application deployment and > accelerate your shift to cloud computing. > http://p.sf.net/sfu/novell-sfdev2dev > _______________________________________________ > Xcsoar-user mailing list > Xcsoar-user@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/xcsoar-user > ------------------------------------------------------------------------------ Start uncovering the many advantages of virtual appliances and start using them to simplify application deployment and accelerate your shift to cloud computing. http://p.sf.net/sfu/novell-sfdev2dev _______________________________________________ Xcsoar-user mailing list Xcsoar-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/xcsoar-user
