A method for determining an enclosing surface of a cavity of an organ of a patient includes receiving a plurality of data points including respective position measurements in the cavity. A first iterative process is performed, that progressively divides sub-volumes into smaller sub-volumes, until a density of the data points in each sub-volume falls below a predefined density. A second iterative process is performed, that progressively discards sub-volumes that are fully-surrounded by populated sub-volumes, and discards the data points contained in the discarded sub-volumes. The enclosing surface of the cavity is defined based on the data points remaining after the second iterative process. The enclosing surface of the cavity is presented to a user.