On Mon, 2005-06-20 at 18:06, Roberto Inzerillo wrote: > > If UTM33N really is 550m displaced from WGS84 at that location, you can > > probably still use the UTM33N data if you offset it by the right amount. > > You may have to rotate it a bit too. Over such a small area, any ground > > distances you measure will be the same in both systems after simple > > correction. > > Sorry, I really don't understand you here. UTM33N and WGS-84 cannot be > displaced in any locations; one is the coordinate system and the second is > the datum. What do you mean with that?
UTM33N is indeed a coordinate system, probably the system of choice for use in Italy, most of the country being in the 33rd 6-degree wide strip of the planet, and in the northern hemisphere: http://www.cnr.colostate.edu/class_info/nr502/lg3/datums_coordinates/utm.html WGS84 is also a coordinate system - the "native" system used by GPS and (unless I'm mistaken) the system used by FlightGear. http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap2/214.htm The difference is that UTM33N (or any of the other UTMs) are intended merely as map projections and only work over a small strip of the world's surface. WGS84 works over the whole planet. You can convert between projections obviously. One of the "complete" ways of doing it is called the Molodensky transform: http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html ( In practice, for small areas, you can assume that just applying an X and Y correction factor and a small rotation about that point will do it. ) > > > Anyway, do you think is possible that apt.dat is that wrong (550m)? Scenery > files for Europe are not precise at all. Are airport locations affected by > the same kind of errors? Maybe because of not enough precise source > informations? > Someone else pointed out that airport data comes from published figures, but it is possible that some airports published their coordinates in a non WGS84 system but someone forgot to do the Molodensky transformation on them before adding them to the apt.dat files. I have noticed though that *scenery* features can be off by a few hundred metres. They are derived in many cases from radar mappings made from space. I live on the coast, and one of these radar maps(*) I once tried to use for a mapping project had the coastline wrong by a hundred metres or more. Steve (*) IIRC, I derived it from FG scenery files from the then-current FlightGear 0.5.6 or similar vintage code. I've not checked the more recent maps of my area for absolute accuracy of the coastline.... Note to self: must do that sometime. :-) _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
