First, some background information. Suppose we are up in the air,
10 nm west of KXYZ airfield (which is colocated with the XYZ vortac).
1) If we were inbound to the field, I would report our position
as 10 nm west, inbound on the 090 radial.
2) If we were outbound from the field, I would report our position
as 10 nm out on the 270 radial.
3) If we were flying northbound, I would report our position
as 10 nm out on the 270 radial.
4) If we were flying northbound, I would report our position
as 10 nm out on the 270 radial.
5) Every enroute chart I've ever seen specifies the location of
waypoints in terms of distance and bearing /from/ the station.
6) Every GPS I've seen lets you enter waypoints in terms of distance
and bearing /from/ some reference.
7) Every NOTAM I've ever seen, when specifying bearings, uses bearing
/from/ some reference.
To summarize: With rare exceptions, locations are specified using the
bearing /from/ the reference.
Also note that in all cases these are /magnetic/ bearings.
So ... I recently used the location-in-air popup menu for the first time.
I selected a distance of 25 nm and an "azimuth" of 270. I had no doubt
that this would put me 25 miles west of the station.
Imagine my astonishment when it put me 25 miles east of the station!
An additional nuisance was that this location was /true/ east not
magnetic east.
Just to add to the excitement, this put me below ground level, in
the mountains east of the station. But that is only tangential
to the present discussion.
One final remark: Pilots talk about radials, courses, and bearings.
The word "azimuth" is not used much. It's a perfectly fine word,
and certainly has its place in the /internals/ of a flight simulator.
OTOH there are lots of users (including me) who want the user interface
(i.e. pilot interface) to be as realistic as possible.
The existing location-in-air popup is shockingly unrealistic from a
pilot's point of view. This needs fixing. The tricky part is fixing
it in a way that doesn't break legacy usages.
I suggest we start by making the following distinctions explicit:
--true-bearing-to
--true-bearing-from
--mag-bearing-to
--mag-bearing-from
Currently, the command-line interface implements --azimuth, which is
documented to be "to" the reference, and is apparently equivalent to
--true-bearing-to. That's OK with me as far as it goes.
1) I suggest that the command-line interface be extended to support
the four explicit bearings itemized above. We can continue to support
--azimuth as a fifth option, synonymous with --true-bearing-to, but
it should be mildly deprecated on the grounds of ambiguity.
2) I suggest that the location-in-air popup be revised so that it
uses the equivalent of --mag-bearing-from, not --azimuth.
As part of this, I suggest changing the wording on the popup from:
Azimuth (deg): ___
to
Bearing: ___° mag from
I don't think this will break any of the documentation, because
AFAICT the location-in-air popup isn't documented in any detail
anywhere.
I'd be happy to try implementing this myself, but I would appreciate
some help or at least hints.
*) I've already changed the appearance of the popup. Easy.
*) I spelled out "deg". I tried putting the ° symbol in the
xml file, but it complained of a parse error. Using °
didn't work, either. Any suggestions on how to encode symbols?
*) I tried putting <offset>180</offset> and
<offset>/environment/magnetic-variation-deg</offset> commands
in the location-in-air.xml file, but the system appeared to just
shrug them off. No effect. What's the trick?
============================================
Has this issue come up before? I didn't see any sign of it.
I searched for
http://www.google.com/search?q=flightgear+location-in-air
and found nothing informative. I also search the flightgear-devel
list at sourceforge and found nothing informative ... just terse
routine cvs log entries.
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