The invention relates to a system (1) for providing an electrical activity map of the heart by means of electrical signals acquired by a plurality of surface electrodes (9). A surface electrodes positions determination unit (4, 6, 13) determines positions of the plurality of surface electrodes by means of optical shape sensing localization. The optical shape sensing element may comprise a wand (4), or alternatively an optical shape sensing fiber embedded in the vest comprising the surface electrodes. The position of a cardiac structure may be determined using ultrasound. An electrical activity map determination unit (16) determines the electrical activity map at the cardiac structure based on the measured electrical signals, the determined positions of the plurality of electrodes and the position of the cardiac structure, in particular, of the epicardial surface. Since optical shape sensing is used for determining the positions of the plurality of surface electrodes and not, for instance, x-rays, the electrical activity map can be determined, without necessarily applying an x-ray radiation dose.