On Tuesday 09 November 2004 15:38, Curtis L. Olson wrote:
> Gerhard Wesp wrote:
> >Hmm...  About what resolution are we talking here?
> Generally, 90m SRTM.
> >What additional data do you have available for the runways? 
> > I guess you have it's position (two endpoints?  center
> > point, direction, length?) and ``elevation''?  Commonly, the
> > runway elevation at both thresholds is given in the approach
> > plates, is this data available?
> Often, elevation of each touch down point is available,
> although that is not included in Robin's database.
> >How about the following KISS solution:  Do a first- or
> > second-order approximation of the elevation of the
> > centerline (least squares). Define the runway using its
> > width, this centerline and no sideways slope.
> This [wc]ould cause problems in places where runways intersect
> and doesn't account for the surface of all the taxiways and
> the rest of the airport, and it doesn't help blend the airport
> cleanly into the surrounding terrain.
> For what it's worth, I believe I have beaten the nurbs
> approach into submission (mostly) and I hope that the next
> scenery release will be an incremental improvement over the
> last one, with better surface matching to the underlying dem,
> but with fewer odd artificats (I believe I have eliminated
> those 5m drop offs that occured in the middle of a very few
> runways.)
> Regards,
> Curt.

Hello Curt,

I might be a bit late on this one - you say you've beaten the 
problem now.

I've used nurbs and splines quite a lot in 3d modelling but don't 
have any programming experience with them.

First of all, when I use them for modelling I don't normally 
expect the resulting curves to pass through the knots/control 
points.  However, with the software I use I can 'invert' the 
control points so that the curve _does_ pass through them.  The 
difference is like drawing a circle within a square or regular 
polygon, so that the curve touches the sides, or outside the 
square or polygon, so that it touches the points.

I'd imagine that the data you've got - an array of x,y,z points -  
would be treated as the control points for the curves and so I 
wouldn't expect the curves to pass through them.

It's also possible to use double or triple points to control the 
'sharpness' of the curve.  They'd also commonly be used along 
the edges of a nurb or spline patch to make sure the patch 
actually extends to the edges.

Dunno if that's any help - it's one of those things that are 
easier to do than explain.


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