Observations of magnetospheric boundaries can provide useful information on the time-dependent dynamics of the coupled solar wind-magnetosphere-ionosphere system. We use an archive of Super Dual Auroral Radar Network (SuperDARN) data to perform a statistical analysis of two different ionospheric boundaries and test their applicability as a proxy for the open-closed field line boundary (OCB). The first is the boundary between eastward and westward flow, the so-called convection reversal boundary (CRB). The second is a new boundary that we have derived that follows the location of maximum curvature across successive convection streamlines (the convection curvature boundary, or CCB). The CRB has previously been shown to be a useful proxy for the OCB under certain circumstances, but we find that unchecked, this technique can identify unphysical boundary locations at a variety of local times, especially close to noon and midnight. The CCB appears to be more robust, identifying a more physical boundary at most local times. We investigate the behaviour of the CCB under differing solar wind driving and geomagnetic conditions. We also compare our results to observations of the poleward auroral boundary latitude (PABL), a well-established proxy for the OCB, made by the Far Ultraviolet (FUV) instrument on the IMAGE satellite. We find that the CCB location is dependent on radar data coverage, but that with sufficient data (60 measurements within a 1 h MLT bin) the CCB provides an extremely good proxy for the PABL at dusk and dawn to within 1-degree latitude.