Re: Channel view hpr settings

New Message Reply Date view Thread view Subject view Author view

Jay Gischer (gischer++at++knex.engr.sgi.com)
Tue, 9 Feb 1999 09:25:28 -0800 (PST)

Good Heavens! I didn't mean to suggest that the situation in any
computer program should be this complicated, only that if you used
(Longitude, Latitude, Altitude) as your coordinate system, things got
very messy. I was trying to point out that the intuition that the
real world has trained us in is not very suitable, as Craig
highlighted. (He and his colleagues had to change the sign on heading to
get things to work out.)

I should have included a :-)

-j

Craig Pepper writes:
> At 09:31 8/02/1999 -0800, Jay Gischer wrote:
> >
> >Since Heading increases in a clockwise direction (from N to E),
> >this is consistent with common navigation in the Western Hemisphere,
> >with +X being West and +Y being North. But in the Eastern Hemisphere,
> >+X is East...
> >
> >To complicate matters further, in the Northern hemisphere +Y is North,
> >but in the Southern Hemisphere +Y is South. So on two separate
> >quarters the earth the coordinate system is right-handed:
> >(+X = N, +Y = W) (N.America) or (+X = S, +Y = E) (S.Africa)
> >and on the other two quarters it's left-handed:
> >(+X = N, +Y = E) (Most of Europe) or (+X = S, +Y = W) (Australia, S. America)
>
> Well, not neccessarily. It seems to me that choice of coordinate system
> axes is just a convention (compare OpenGL Y up, Performer Z up, and the Z
> down system used by our aero guys). Choosing different axis directions
> depending on the hemisphere you're in would be a pain to implement and
> fairly inefficient. Either that or you'd fall out of the sky as you passed
> over the equator (and it's not even the year 2000 yet :-).
>
> We've implemented a double precision spheroid (lat, long, altitude)
> coordinate system that resolves to a right-handed float coordinate system
> around an origin tangential to the spheroid, with +Z up, +X east and +Y
> north. This fits in with pf coordinates and works fine all over the globe,
> and note that we do most of our flying in the southern/western hemisphere.
> Our coordinate system does have to reverse the sign of compass headings (0
> north, increasing clockwise) to make them pf friendly (0 +Y, increasing
> counterclockwise).
>
> Just hoping to head off any confusion that might result from the previous
> post.
>
>
> -----------------------------------------------------------------------------
> Craig Pepper Home: craigp++at++matra.com.au Work: cpepper++at++syd.csa.com.au
> Visual Simulation Development
> CSC Australia Pty Ltd

New Message Reply Date view Thread view Subject view Author view

This archive was generated by hypermail 2.0b2 on Tue Feb 09 1999 - 09:25:33 PST

This message has been cleansed for anti-spam protection. Replace '++at++' in any mail addresses with the '@' symbol.