Exactly. That's the point. Wouldn't it be desireable to have xcsoar handle the final glide (and all glide calculations) with the physically presice GPS-Altitude and at the same time (!) handle Airspaces with pressure altitude? I don't see a technical reason, why it has to be either GPS or pressure Altitude for both. What do you think?
Henrik Am 02.10.2010 00:48, schrieb Rob Dunning: > US contests use pressure altitude as the primary measurement for > altitude. From US FAI Contest Rules: > > 6.7.3.3 If a Flight Recorder records both calculated and pressure > altitude, pressure altitude will be the primary data source and > calculated altitude will be the backup data source for flight evaluation. > > However, depending on the pilot and the day, either GPS or pressure > altitude may be preferable for the glide computer (either final glide > or for airspace violations). > > XCSoar has an option in its configuration to use pressure altitude, > but defaults to GPS altitude. > > Rob > > At 05:36 AM 9/30/2010, Henrik Bieler wrote: >> 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
