Medical apparatus includes a probe with a basket assembly at its distal end, including a plurality of resilient spines with multiple electrodes arrayed along a length of each of the spines. Processing circuitry is configured to acquire a first bipolar electrical signal from the tissue between first and second electrodes at first and second locations along a first spine of the basket assembly, and to acquire a second bipolar electrical signal from the tissue between the first electrode and a third electrode in a third location on a second spine of the basket assembly, and to interpolate, based on the first and second bipolar electrical signals, a vectorial electrical property of the tissue along an axis that passes through the first location and between the second and third locations.