A method includes receiving, from a probe that includes electrodes and is positioned inside a cavity in an organ of a patient, (i) proximity signals indicative of proximity of the electrodes to a wall of the cavity, and (ii) position signals indicative of positions of the electrodes within the cavity. Based on the proximity signals and the position signals, at least a portion of a volume of the cavity is represented by a sphere model including multiple spheres. A direction is identified along which one or more spheres are larger than one or more surrounding spheres by at least a given factor. Based on the indicated direction, a location of an opening in the wall of the cavity is estimated and presented to a user.
