Indeed. We are currently running atmosphere code written by John Denker, et al, that takes lapse rate into account. The reason pressure-sea-level-inhg doesn't match the altimeter setting is because it doesn't in real life. The altimeter setting number is chosen so the altimeter reads the field elevation when on the field. As David points out, if the sea-level temperature is not 15 degC. In your example[1]:
2010/05/29 15:54 KIND 291554Z 36004KT 10SM SCT250 27/18 A2994 RMK The local temperature is 27 degC (METAR) and local pressure is 28.9367 (/environment). KNID is 797 ft. MSL [2] and I assume your tower is 50 feet high, so local altitude is 847 ft. The equation for altimeter setting (QNH) is: QNH = (local_pressure^0.1903 + 1.313e−5•local_elevation)^(1/0.1903) [3] So: 29.84 = (28.94^0.1903 + 1.313e−5•847)^(1/0.1903) If you were in the UFO and descended to sea-level you should see /environment/pressure-inhg == /environment/sea-level-pressure-inhg when /position/altitude-ft == 0 Thanks, Ron [1] http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg [2] http://www.airnav.com/airport/KIND [3] http://www.av8n.com/physics/altimetry.htm#eq-qnh Other links on this subject: http://www.aviationbanter.com/showthread.php?t=16985 http://www.airliners.net/aviation-forums/tech_ops/read.main/44629/ On Saturday 29 May 2010 12:24:19 David Megginson wrote: > It's been a long time, but in addition to any issues with your > altimeter setting I *think* I might have added code ~8 years ago to > simulate the effect of temperature on altimeter readings. In real > life, even if you have exactly the correct altimeter setting for the > ground below you, and the altimeter is perfectly calibrated, your > altitude can be off by hundreds or thousands of feet from what you see > on the altimeter. > > Generally speaking, you'll be lower than you think if the temperature > is below ISA, and higher than you think if the temperature is above > ISA, because temperature affects how much the air pressure changes > with altitude. The altimeter is calibrated to assume 15 degC at sea > level and a perfect consistent temperature gradient above that. > > > All the best, > > > David > > > In Canada, we're required to increase minimum altitudes for > approaches, etc., when temperatures are cold. We also have designated > mountainous regions where you have to fly higher in the winter. > > On Sat, May 29, 2010 at 1:02 PM, Rob Shearman, Jr. <rmsj...@yahoo.com> wrote: > > Hi there -- > > > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > > noticed there were some significant discrepancies between the displayed > > altitude and the altitude found in the property tree at > > /position/altitude-ft. I was able to observe the same problem in the > > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > > altitude at close to 35000. So the discrepancy at cruise is as much as > > 1000-2000 feet sometimes, even when using the altimeter setting reported > > in the METAR (which, of course, you're not supposed to do above 18,000 in > > the U.S., but for testing purposes I did so to see if it accounted for > > the error). > > > > However, today loading up my custom ATC scope I observed a discrepancy in > > altimeter readings which may account for the problem. > > > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > > > Notice in the shot that the METAR is reporting 29.94, but the property > > tree and the scope panel are arriving at 29.90. I presume at high > > altitude this discrepancy would account for differences in the gauge > > reading. > > > > This is with the 25 April CVS build, so if altimeter code has changed > > since the migration to git, I apologize in advance. > > > > Thanks, > > -R. (MD-Terp) > > > > Robert M. Shearman, Jr. > > Transit Operations Supervisor, > > University of Maryland Department of Transportation ------------------------------------------------------------------------------ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
