Methods, systems, and apparatus for determining anatomical orientations are described. Data is received that indicates a position of a reference device configured to move with a pelvis of a subject. Data is received that indicates locations corresponding to a reference surface. Data is received that indicates locations on the pelvis. One or more anatomical orientations are determined based on the locations corresponding to the reference surface and the locations on the pelvis. The anatomical orientations are registered to the reference device.
