Stefan, I saw your examples on the right showing parabolas with refined control vertexes... So I just refined with splinedit main CV with weight 1.0 to 2 new control vertexes and parabolas matches (level = 0 equal level = 10)... But now I don't know what shell we do with your ellipse? Using A2008, so I can't explode it to spline... Maybe that's just what it needs to be exploded into spline. Check my *.dwg now...
Sorry, for posting wrong *.dwg, it seems that after saving CAD didn't represented spline as it did just after splinedit - refine... So all I could do is splinedit from A2011 - A2012 and convert it to polyline with precision 99. Results are approximately correct... Still I don't know how you even draw that ellipse, but I read somewhere on www that if you draw it with variable PELLIPSE set to 1, you can draw it as polyline, and another method is to save dwg as DXF R 12 and after do DXFIN you'll get 3dpolyline from your ellipse, but I've tried this and I think that approximation is bad and for ellipse and for spline-parabola, and the third method is to offset ellipse for some amount and then offset it back - ellipse will be converted into spline (I offset your ellipse by 1.0 inside, splinedit -> polyline precision 13, and offset new polyline by 1.0 outside)... So in the end I'll post what I did best to simulate approximation of representation of extension level = 10 spline-parabola and approximation for your ellipse...
M.R.