Re: [Opengc-devel] nav display status

[Damion S. <be...@cs...>]

Thanks for the reply,

I've been doing some reading today (I find this an interesting problem) 
and came up with the following information:

According to http://mathworld.wolfram.com/topics/MapProjections.html 
the Lambert conformal conic projection does not display either great 
circles (orthodromes) or lines of constant heading (loxodromes) as 
straight lines. Apparently, one of the few projections where 
orthodromes are straight lines is the gnomonic projection, but it's 
nonconformal (does not preserve small angular relationships) and 
results in rather extreme distortion away from the center of the 
projection.

The Mercator projection plots loxodromes as straight lines. One web 
site on navigation ( 
http://www.psych.usyd.edu.au/vbb/woronora/maritime/Navigation.html ) 
indicates that a common trick is to break great circle routes into 
segments of loxodromes to make executing the route easier (to avoid 
having to steer a continuously variable heading). This trick also 
applies to navigation using airways, since they generally (always, 
perhaps? I'm not familiar enough to say) connect two nav beacons or 
intersections with a line of constant heading.

So, it seems that a consequence of displaying great circles/orthodromes 
as straight lines is an inability to plot loxodromes as straight lines. 
This would result in a route planned using airways to appear curved 
despite the lines being of constant heading. Is this the case on an 
actual ND? As I understand it, additional constraints such as ETOPS 
certification, weather, and so on often preclude the use of 
great-circle routes in any case.

Whew. I think I'm about done with math for today... this is not exactly 
light reading. Any corrections or comments are encouraged.

Cheers,
-Damion-

On Friday, May 9, 2003, at 11:09 AM, Manuel Bessler wrote:

> On Wed, May 07, 2003 at 02:38:06PM -0400, Damion Shelton wrote:
>> In any case, do any of you know how this is handled in the real world?
>> There are of course other projections available, some of which I have
>> the math for and some which I don't. On the scale that navigation
>> occurs (as distinct from flight planning) it's mostly safe to assume
>> that the world is flat, but I'd like to have a reasonably correct
>> implementation.
>
> Having done some research for my own project earlier this year,
> I believe that the projection used on current Nav Displays is a Lambert
> conformal conic projection. This also seems to be the most common
> projection used for aviation paper maps.
>
> I talked to a real world Airbus Pilot about that, and although he could
> not tell me which projection is used, he confirmed my thoughts. (eg.
> great circles are straigt lines on the ND, except some older Airbusses)
> This pointed my to believe that the above mentioned projection is what
> he stares at in his "office".
>
>
> Regards,
> Manuel

---------
Damion Shelton
Carnegie Mellon University, Robotics Institute
A408-o Newell Simon Hall
412.268.3866 (office)
412.818.8829 (cell)
http://www.cs.cmu.edu/~beowulf
---------
Here's a good thing to do if you go to a party and you don't know 
anybody: First take out the garbage. Then go around and collect any 
extra garbage that people might have, like a crumpled napkin, and take 
that out too. Pretty soon people will want to meet the busy garbage guy.