Sets the value of StartRadiusOfCurvature attribute. For a NONLINEAR horizontal segment type the radius of the curve is at the start point (Placement of the segment). For CIRCULAR type it is constant i.e. StartRadiusOfCurvature and EndRadiusOfCurvature are always the same. For LINE type, both StartRadiusOfCurvature and EndRadiusOfCurvature are 0. If the radius is 0 it is interpreted as INFINITE. Positive values imply a CCW direction whereas negative CW.