Abstract: An apparatus assigns to a plurality of grid areas of a grid, the grid defined to virtually cover a positioning site, a probability that an object is located in the respective grid area. The probabilities are based at least on characteristics of wireless signals propagating in at least one direction between the object and devices distributed at the positioning site. The apparatus selects grid areas such that the total probability assigned to the selected grid areas at least reaches a predetermined probability. The apparatus determines a set of at least one virtual room, with the set containing selected grid areas, and provides an indication of the set of at least one virtual room as an estimated location of the object.