A computer that determines at least an anatomic feature of a left atrial appendage (LAA) is described. During operation, the computer generates a 3D image associated with an individual's heart. This 3D image may present a view along a perpendicular direction to an opening of the LAA. Then, the computer may receive information specifying a set of reference locations. For example, the set of reference locations may include: a location on a circumflex artery, a location between a superior portion of the LAA and a left pulmonary vein, and/or a location on a superior wall of the LAA and distal to trabeculae carneae. Next, the computer automatically determines, based, at least in part, on the set of reference locations, at least the anatomical feature of the LAA, which is associated with the opening of the LAA and a size of a device used in an LAA closure (LAAC) procedure.