Medical apparatus, comprising: a probe, comprising: an insertion tube configured for insertion into a body cavity of a patient; and a basket assembly connected distally to the insertion tube and comprising a plurality of resilient spines having respective proximal and distal tips and comprising multiple electrodes arrayed along a length of each of the spines between the respective proximal and distal tips, such that the proximal tips of the spines are joined at a proximal end of the basket assembly, and the distal tips of the spines are joined at a distal end of the basket assembly, and the spines bow radially outward when the basket assembly is deployed in the body cavity, whereby the electrodes contact tissue in the body cavity; and processing circuitry, which is configured to: acquire a first bipolar electrical signal from the tissue between first and second electrodes (48a, 48b) at first and second locations along a first spine of the basket assembly, 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 interpolate, based on the first and second bipolar electrical signals, a vectorial electrical property of the tissue at the location of the first electrode and in a direction along an axis that passes through the first location and between the second and third locations. a probe, comprising: an insertion tube configured for insertion into a body cavity of a patient; and a basket assembly connected distally to the insertion tube and comprising a plurality of resilient spines having respective proximal and distal tips and comprising multiple electrodes arrayed along a length of each of the spines between the respective proximal and distal tips, such that the proximal tips of the spines are joined at a proximal end of the basket assembly, and the distal tips of the spines are joined at a distal end of the basket assembly, and the spines bow radially outward when the basket assembly is deployed in the body cavity, whereby the electrodes contact tissue in the body cavity; and an insertion tube configured for insertion into a body cavity of a patient; and a basket assembly connected distally to the insertion tube and comprising a plurality of resilient spines having respective proximal and distal tips and comprising multiple electrodes arrayed along a length of each of the spines between the respective proximal and distal tips, such that the proximal tips of the spines are joined at a proximal end of the basket assembly, and the distal tips of the spines are joined at a distal end of the basket assembly, and the spines bow radially outward when the basket assembly is deployed in the body cavity, whereby the electrodes contact tissue in the body cavity; and processing circuitry, which is configured to: acquire a first bipolar electrical signal from the tissue between first and second electrodes (48a, 48b) at first and second locations along a first spine of the basket assembly, 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 interpolate, based on the first and second bipolar electrical signals, a vectorial electrical property of the tissue at the location of the first electrode and in a direction along an axis that passes through the first location and between the second and third locations. acquire a first bipolar electrical signal from the tissue between first and second electrodes (48a, 48b) at first and second locations along a first spine of the basket assembly, 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 interpolate, based on the first and second bipolar electrical signals, a vectorial electrical property of the tissue at the location of the first electrode and in a direction along an axis that passes through the first location and between the second and third locations.