A processor is configured to construct a baseline impedance model (BIM) that models a portion of a heart of a subject as a collection of three-dimensional cells, each of which corresponds to a respective volume within the heart, at least some of the cells being designated as baseline-impedance cells, for each of which the BIM specifies a respective baseline impedance, to ascertain, based on a signal received via an electrical interface, an impedance between a catheter electrode, which is within the heart, and an external electrode that is externally coupled to the subject, to identify one of the baseline-impedance cells as a reference cell, to ascertain that the catheter electrode is within a threshold distance of tissue of the heart, by comparing the ascertained impedance to the baseline impedance that is specified for the reference cell, and to update a map of the tissue in response to the ascertaining.