Re: [Flightgear-devel] Carrier Altitude

[Curtis Olson]

Quick explanation: the world is curved (oblate spheroid) so if in order to
have an ocean that measures zero MSL at all points, it would have to be
curved.  To do this perfectly requires a *lot* of polygons.  We have been
using large polygons for the ocean so that leads to some errors depending on
where you are within the polygon.  Near the verticies will be pretty
accurate, near the middle could be off by a few meters.

Regards,

Curt.



On Tue, Jan 25, 2011 at 11:57 AM, Peter Brown
<smoothwater...@adelphia.net>wrote:

> In attempting to place an item on the ocean surface, I came to realize that
> it's not the Nimitz that is "hovering" above MSL, it's that the "ocean
> surface" is about 7 meters *below* MSL.  I was going to suggest simply
> dropping the carriers to match, but then looking around I discovered that it
> was not constant, as the ocean surface is different from front to back of
> the Nimitz.
>
> So the ocean surface is not flat.  Where it meets the terrain north of the
> Golden Gate Bridge the water is at zero MSL.  It then slopes down as it you
> head out to sea, to -8.72 meters below sea level, before starting to come
> back up again.  I did not continue out to see if it continues to rise and
> fall, or if it's purely a dip in this area.  This is different from the
> ocean surface being curved to match the earth, as it actually has a dip, at
> least in this spot.
>
> See screenshots -
>
> Terrain intersection north of Golden Gate
> [URL=
> http://s512.photobucket.com/albums/t325/barefootr/?action=view&current=Screenshot2011-01-24at102804PM.png][IMG]http://i512.photobucket.com/albums/t325/barefootr/th_Screenshot2011-01-24at102804PM.png[/IMG][/URL
> ]
>
> At the Nimtiz
> [URL=
> http://s512.photobucket.com/albums/t325/barefootr/?action=view&current=Screenshot2011-01-24at100253PM.png][IMG]http://i512.photobucket.com/albums/t325/barefootr/th_Screenshot2011-01-24at100253PM.png[/IMG][/URL
> ]
>
> West of Nimitz
> [URL=
> http://s512.photobucket.com/albums/t325/barefootr/?action=view&current=Screenshot2011-01-24at103120PM.png][IMG]http://i512.photobucket.com/albums/t325/barefootr/th_Screenshot2011-01-24at103120PM.png[/IMG][/URL
> ]
>
> Anyone know why?  Or, more importantly, can we set the carriers at an
> altitude appropriate for their default location, so they are no longer
> "hovering"?  I'd prefer to do it globally so everyone was at the same
> altitude.
>
> Thanks,
> Peter
>
>
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-- 
Curtis Olson:
http://www.atiak.com - http://aem.umn.edu/~uav/
http://www.flightgear.org -
http://www.flightgear.org/blogs/category/curt/<http://www.flightgear.org/blogs/category/personal/curt/>

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