Abstract
The ellipse angle is examined to obtain numerical values of roundness of curve. Similarly to the ellipse angle the hyperbola angle is introduced also to express the roundness values of curve. An arc is matched with a similar shape of conic section by examining changes of radiuses of curvature along the curves. The angle value of the selected conic section can represent the roundness value of the arc. Changes of radiuses of curvature of conic section can completely cover those of all types of curves. It shows a fundamental characteristic of the conic section. The roundness values are applied to four curves of fundamental functions.
