On Wed, 2005-06-22 at 17:05, Roberto Inzerillo wrote:
> Hi Steve,
> > 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:
> I agree.
> > WGS84 is also a coordinate system - the "native" system used by GPS and
> > (unless I'm mistaken) the system used by FlightGear.
> Sorry, but I thought WGS84 is a Datum, one of the possible geometric
> ellipsoid to choose from for describing the earth surface.


> It's not a
> coordinate system by it's own. Right?

No - it is a coordinate system as well. "WGS84" is in some ways a bit
confusing as it defines two things: a spheroid (known as the WGS84
spheroid, apparently) and a reference system on top of that (i.e an
equator and a prime meridian). The reference system doesn't seem to have
a name other than maybe "the WGS84 reference system :-)).

But WGS84 is only any use for describing points on a spheroid. If you
want to make paper maps of any area of the world you need a projection
system. Italy (evidently) uses UTM33N.

>  This decision changes the way the
> normal in a point is defined as well as the point considered to be the
> center of the earth, so the latitude value depends on this choice too.
> What am I missing?

Nothing! Read on....

> I visited www.atlanteitaliano.it (a Government web site); there I find a
> complete map system of Italy. It's a ECW driven interactive site. I can pick
> a point and have the coordinates back. www.atlanteitaliano.it says (for the
> region where LICP lays) Datum is WGS84 and Projection is NUTM33 (UTM, region
> 33, North).

This is interesting. It implies that the data points coming from that
web site are originally WGS84 already. If the site gave you lat/long
then you could forget the UTM33 knowledge - lat/long would be the same
on both.

However, your comments below show that the coords given on the site are
in UTM33 Eastings and Northings, and you'll have to convert them.

> Airport LICP start and end points of the runway are:
>    Coordinates(1)
>  - Point A
>      X: 352091.5   Y: 4220579.4
>  - Point B
>      X: 352213.2   Y: 4219364.1
> Knowing Datum, Projection and Coordinates is enough for determining the
> point in the space, right?

Yes I think so. Looks like point A is 353.0915km east and 4220.5794km
north of the UTM33N reference point.

> - Second step (optional): coordinate conversion
> I make use of "Geographic Translator Version 2.2.5" for getting the
> conversion, which is usefull for further data verification.
> The source data are the above ones; I specify that Datum is "WGE: World
> Geodetic System 1984" (Ellipsoid is "WE: WGS84"), "Universal Transverse
> Mercator (UTM)", Zone is 33, Hemisphere is North.
>    Coordinates(1)
>  - Point A
>      Easting/X(m): 352091.5   Northing/Y(m): 4220579.4
>  - Point B
>      Easting/X(m): 352213.2   Northing/Y(m): 4219364.1
> I asked Geographic Translator to give me the output so that Datum is "WGE:
> World Geodetic System 1984" (Ellipsoid is "WE: WGS84"), Geodetic.
> I get the following output:
>    Coordinates(2)
>  - Point A
>      Longitude: 13 18 45.5E   Latitude: 38 7 15.4N
>  - Point B
>      Longitude: 13 18 51.4E   Latitude: 38 6 36.1N
> These should be the coordinate values of the two points in FG Scenery,
> right? Well these are not!

OK, so either "Geographic Translator Version 2.2.5" is buggy, or the
published UTM33N eastings and northings are wrong.

> - Third step: aerial picture of LICP overlap to FGFS scenery LICP
> I make use of FlightGear Scenery Designer for importing the aerial picture
> and overlap it to the scenery so to complete it with surrounding buildings,
> roads and so on. I make use of the tool called "Map Fragments". Here I can
> import a picture of the terrain, fixing the coordinates of three known
> points.
> www.atlanteitaliano.it gives me those three points (two of them being the
> two above described, and choosing a third usefull point).
> Well, here FGSD asks me which kind of coordinates I am importing. I set
> "System: Universal Transverse Mercator", "Zone: 33", North, "Datum: WGS-84".
> For the two points, I use the Coordinates(1).
> Well, the problem is when I import this "MAP Fragment" into the scenery. The
> result is the picture of the airport being translate by almost 500m north to
> the position of the airport of the scenery.
> The scenery airport coordinates, as viewed into FGSD, are:
> Coordinates(3)
>  - Point A:
>      E 013:18'50.4   N 38:06'18.0
>  - Point B:
>      E 013:18'45.8   N 38:06'57.6
> The error is that (Coordinates(3)A.N)!=(Coordinates(2)A.N) .
> I get the same error for PointB too, of course.

Yeah, I see that. So potentially FGSD is doing a dud conversion too. At
least we have the source code for FGFS and can check it....

> The two runways are perfectly parallel to each other. There seems not to be
> any rotation error.
> So now what's the mistake? Am I doing something wrong? Is FGSD doing
> something wrong? Is atlanteitaliano.it reporting wrong coordinates? Is the
> apt.dat wrong?

Or all of them! :-)

> I checked other spots of the city around, some seem correct enough. The
> overlapping is not very precise because the scenery is not high definition,
> so I have to guess if a coastline seems correct enough because of correct
> source data or just because of luck; anyway I think the error is just with
> this LICP airport and not with the rest of the scenery territory around.

That sort-of says that FGSG is doing the conversion correctly.

This could be a hold-over from the Cold War. In many countries (not
Britain though) airports are shared resources with the military. British
aircraft enthusiasts have been known to end up in jail for not realising
that taking photos in foreign airports can count as spying! (Like in
Greece, not long ago).

Locations of airports could well be considered "classified information"
in these countries. Is this true of Italy? You may be seeing a
deliberate mistake here....

Take a look a British "Ordnance Survey" map, and try and find the R.A.F
bases. They aren't there(*). Just fields usually. Civilian airports are
never shared with the military over here, so they show up.

Oh, and if you publish the fixed coordinates, you might get a visit from
the police...  I'd like to be joking here, but it took a lot of
diplomacy and many months of hard work to get those British
aircraft-spotters out of jail in Greece. Be careful please. The cold war
is over, but politicians change their ideas slowly.


(*) Actually, this may have changed in the last five years. It depends
on the issue date for the map.

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