Please help on understanding arc/circle

Hi,

I use TopExp_Explorer to get the edges of a selected shape. Then I use BRep_Tool::Curve to get the geometric element of an edge. The upper listbox from the screenshot shows the edges and the lower listbox shows the properties, I got from geometric element.

From the selected face, both vertical lines have the same X-distance but with opposed sign, so the center of the hole is X0. I expected the arc to be a Geom_TrimmedCurve based on a circle, but it is a Geom_Circle. The "parameters" in the lower listbox are first and last from BRep_Tool::Curve. Radius, center and direction of a circle are ok so far.

My understanding suffers on understanding parameter values. I suppose, that an arc needs some angles as parameters, but the upper arc starts at 0 and ends at 35.4, whereas the lower arc starts at 252.3 and ends at 287.7 - difference is both times 35.4

I thought, that from top view every point on a line crossing the center of the circle has the same angle.

So what am I missing?

From the properties of geom_curve (together with first and last) - how can I calculate points on the arc/circle?