"I was always told by the engineers to measure from a datum point. Are these the actual manufacturing drawings or are the simply a guide for after market manufacturers?.Also the curve isn't Conic is curvature continuous maybe at 5 or 7 point or something, haven't worked in alias for years but from the looks of it thats what they're trying to show, in automotive I don't think you even bother marking up this curves as you hand the surface data straight to the manufacturer."