Right -- outside the USA, much of the x-plane airport data is hand entered
and submitted by end-users with no quality control other than people are
welcome to research and fix problems they find as they find them. I
wouldn't be surprised if some of the entries are complete guesses or crazy
typos. Inside the USA we have the FAA data to reference. For a while DAFIF
was world wide, but more recently I think they pulled that back and just
maintain the USA data.
Quality + free GIS data is often hard to find. We are very lucky to have
some of the products we do have ... like SRTM and VMAP.
Curt.
On Thu, Sep 15, 2011 at 8:42 PM, John Denker <j...@av8n.com> wrote:
> On 09/15/2011 05:15 PM, Martin Fenelon wrote:
>
> > I like to think that the positional errors of many (most non US?)
> > aerodromes are due to mistakes made when changing from one datum to
> > another.
>
> Well, that's not what I think, based on looking at
> the data.
>
> The very first non-US example I looked at was
> ES03 "Hova"
>
> for which the apt.dat position is off by hundreds of meters,
> to the southeast. Nearby we have
> ESVF "Frolunda"
>
> for which the apt.dat location is off in another direction,
> and the relationship of its two runways to each other is
> wrong.
>
> It is hard to see how a change in datum could have a different
> effect on two nearby airports ... and there is just no way it
> could have a different effect on two runways at the same airport.
> There aren't that many different datums to play with.
>
> > Errors in runway orientation at unmodifed airfields (with
> > default layouts) appear to be caused by confusion between magnetic and
> > true bearings, with magnetic being used as true.
>
> Uhh, in apt.dat the runway heading for ES03 is off by more
> than 35 degrees. The local magnetic deviation is more like
> 4 degrees.
>
> Bottom line: Many of the apt.dat entries are just wrong.
>
> You don't need any ultra-sophisticated geodetic expertise
> to understand what is going on. The entries are just
> wrong.
>
> If you want something to be accurate to a few centimeters,
> or even a few meters, then some expertise is required, but
> that's not what we're talking about here.
>
>
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--
Curtis Olson:
http://www.atiak.com - http://aem.umn.edu/~uav/
http://www.flightgear.org - http://gallinazo.flightgear.org
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