Sets the start angle of the circular arc in the range ±2PI radians (DXF 50). A positive value specifies the angle measured from the OCS X-axis counterclockwise. A negative value specifies the angle measured from the OCS X-axis clockwise. The initial value is zero by default. 

If an angle value is negative, this method automatically converts it to the positive equivalent. If an angle value is more than 2PI, this method automatically converts it to the range 0 to 2PI.

Example of Working with the Circular Arc 

Specific Circular Arc Properties 

OdDbArc::startAngle() method, OdDbArc::setEndAngle() method