On 09/15/2011 03:08 PM, HB-GRAL wrote: > No, it looks like the mapping with apt.dat data is inaccurate, at least > outside the United States.
The following repeats an email I sent quite a while ago, which somehow seems to have gotten lost: On 09/10/2011 03:54 PM, HB-GRAL wrote: > I am just curious why FlightGear and OSM have the same "accurate" > position, and Google map shows another one. > > I am sure, there are different problems, but some enlightenment will be > greatly appreciated here. Many of the entries in apt.dat are wrong. Large errors are particularly prevalent in the entries for small airports that don't have instrument approaches. In some cases google maps makes the same mistakes. I haven't checked whether openstreetmap makes the same mistakes. If so, the obvious explanation is that all three have copied from each other, or from the same defective upstream source. When I say entries in apt.dat are wrong, I am not comparing against one database of features against another, but rather comparing it against satellite photography and shuttle radar terrain mapping ... which are known to be quite accurate. Indeed the accuracy of the imaging is confirmed by the fact that *some* of the apt.dat entries are spot-on, especially for the major airports. On 09/12/2011 12:56 AM, Alan Teeder wrote: >> Point positions of aeronautical navaids and airfields have mostly been well >> surveyed and their positions should therefore be accurate. Well, maybe they "should" be accurate, but in fact many of the entries in apt.dat are not. Some of the errors are quite large. For example, the location of CO80 "USAF ACADEMY BULLSEYE AUX AIRSTRIP" is off by several hundred meters. As another example, the runway heading for 57AZ "La Cholla Airpark" is wrong by more that 20 degrees. Furthermore, the location and runway heading are wrong for 2B3 "PARLIN FLD". If you want a more-or-less endless supply of errors, look at even smaller, private, and/or unpaved airfields. The database also contains a goodly number of entries for airports that ceased to exist many years ago. These errors have got nothing to do with metaphysical questions about the meaning of "where it is". The entries are just wrong. They've been wrong all along. In most cases version 850 has the same errors as version 810. The metaphysical issues are much, much smaller. As it says at ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT >>> In general the ITRS (and its realizations ITRFyy) are identical >>> to WGS84 at one meter level. Similarly, there are still some people who still use NAD83, which is not unreasonable. It agrees with WGS84 within one or two meters in North America ... and the offsets are well known. The apt.dat errors are huuuuge compared to any such offsets. ========= I have a tool that reads apt.dat and writes kml. Thereupon the kml can be imported into a GIS system such as GRASS or google-earth. This making it easy to compare apt.dat with the satellite and shuttle images. ------------------------------------------------------------------------------ BlackBerry® DevCon Americas, Oct. 18-20, San Francisco, CA http://p.sf.net/sfu/rim-devcon-copy2 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
