From: Henrik B. <hen...@gm...> - 2010-10-02 23:00:23
|
Am 03.10.2010 00:12, schrieb Ian: > On 30/09/2010 11:36, Henrik Bieler wrote: > >> 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. > I think it would be nice if Xcsoar could automatically adjust the QNH > setting to make the measured altitude on the ground the same as the > altitude for the take off location taken from the way point database, or > the terrain model. > > Maybe then it should offer the user an opportunity to correct this (or > to set it if no suitable waypoint or terrain data is available). > Agree. That would make a nice, QNH corrected pressure altitude. >> Remember the old saying: "In Winter the mountains are higher" ;-) >> With GPS-Altitude, you don't get this error. > For terrain clearance, GPS altitude is probably best. > > For final glide it probably makes no difference if you calibrated QNH > before launch (unless your landing airfield is at a very different > altitude to the take off one). > Disagree. As stated in detail in my first mail. The temperature error at altitudes higher than the airfield where the QNH was measured or where the altimeter was set to airfield altitude (which is practically the same) is significant at temperatures other than ISA. 150m like in my example makes a difference at least for my final glide considerations ;-) The air pressure error can be compensated for by setting the correct QNH. However the temperature error is just there. The magnitude depends on the deltaT to ISA conditions and the heigt of the air column between the glider and the Airfield. Therefore it is zero on the ground... growing proportionally with altitude. Therefore I would prefer GPS-Alt for final glide calculations... >> 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. > Airspace defined in height ASL, should be measured with pressure > altitude with corrected QNH. > > Airspace defined in flight levels should be measured with pressure > altitude measured with standard (1013mb) subscale setting. > > This might become an issue in competitions if there are penalties for > airspace violation. > > Agree. Therefore I vote for xcsoar using QNH-press-alt for airspace ASL considerations, and 1013-press-alt. (wich is easily calculated if you have QHN-Alt and QNH) for the FL airspaces. @Max or Tobias: Would it mean a major code change to implement an idependent setting of altitude sources for final-glide-calcs and airspace-calcs? Should I create a ticket? If I can support you with the implementation in any way even though I'm not a programmer let me know! Greetings Henrik > 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 > Xcs...@li... > https://lists.sourceforge.net/lists/listinfo/xcsoar-user > |