An apparatus for obtaining cardiac electrograms comprising: a catheter having an elongated catheter body , the catheter body having an interior, proximal end, a distal end and at least one lumen therethrough; a basket-shaped assembly at the distal end of the catheter body, the basket-shaped assembly having a longitudinal axis, a proximal extremity and a distal extremity and comprising a plurality of splines connected at the proximal extremity and the distal extremity, the splines comprising a plurality of spline electrodes , the basket-shaped assembly configurable in an expanded arrangement wherein the splines bow radially outwardly forming an expanded basket shape and in a collapsed arrangement, wherein the splines are arranged generally along the longitudinal axis of the catheter body ; a distal electrode location sensor mounted at or distal to the distal extremity of the basket-shaped assembly , and a proximal electrode location sensor mounted at or proximal to the proximal extremity of the basket-shaped assembly , whereby, in use, coordinates of the distal electrode location sensor relative to those of the proximal electrode location sensor can be determined and taken together with known information pertaining to curvature of the splines of the basket-shaped assembly to determine locations of each of the spline electrodes ; and a contraction wire for retracting and expanding the basket-shaped assembly , the contraction wire being attached to the distal extremity of the basket-shaped assembly and forming a longitudinal axis of symmetry thereof, characterized in that the apparatus further comprises a far-field electrode disposed in the interior of the expanded basket shaped assembly on the contraction wire; and in that the splines are configured to prevent the far-field electrode contacting cardiac surfaces when the basket-shaped assembly is in the expanded arrangement. a catheter having an elongated catheter body , the catheter body having an interior, proximal end, a distal end and at least one lumen therethrough; a basket-shaped assembly at the distal end of the catheter body, the basket-shaped assembly having a longitudinal axis, a proximal extremity and a distal extremity and comprising a plurality of splines connected at the proximal extremity and the distal extremity, the splines comprising a plurality of spline electrodes , the basket-shaped assembly configurable in an expanded arrangement wherein the splines bow radially outwardly forming an expanded basket shape and in a collapsed arrangement, wherein the splines are arranged generally along the longitudinal axis of the catheter body ; a distal electrode location sensor mounted at or distal to the distal extremity of the basket-shaped assembly , and a proximal electrode location sensor mounted at or proximal to the proximal extremity of the basket-shaped assembly , whereby, in use, coordinates of the distal electrode location sensor relative to those of the proximal electrode location sensor can be determined and taken together with known information pertaining to curvature of the splines of the basket-shaped assembly to determine locations of each of the spline electrodes ; and a contraction wire for retracting and expanding the basket-shaped assembly , the contraction wire being attached to the distal extremity of the basket-shaped assembly and forming a longitudinal axis of symmetry thereof, characterized in that the apparatus further comprises a far-field electrode disposed in the interior of the expanded basket shaped assembly on the contraction wire; and in that the splines are configured to prevent the far-field electrode contacting cardiac surfaces when the basket-shaped assembly is in the expanded arrangement. <EOS>