1. An apparatus for mapping a three-dimensional space in medical applications for diagnostic, surgical or interventional medicine purposes, wherein it comprises: acquisition means capable of recording two-dimensional images of said three-dimensional space from at least a first recording position and a second recording position, the coordinates of said first and second recording positions being not known; and a reference target, movable between a first target point and a second target point of said three-dimensional space, said second target point being located in known position relative to said first target point, said reference target comprising a plurality of marker elements (M 1 , M 2 , M 3 , M 4 ), observable by said acquisition means, said marker elements being mutually positioned according to a predefined geometry, in such a manner as to define a three-dimensional reference system (B Rif ), when said reference target is positioned in said first target point; and a processing unit adapted to receive data indicative of a first image and of a second image of said three-dimensional space, said first image being acquired by said acquisition means in said first recording position with said reference target positioned in said first target point in such a manner as to be framed by said acquisition means in said first recording position, said second image being acquired by said acquisition means in said second recording position with said reference target shifted in said second target point in such a manner as to be framed by said acquisition means in said second recording position, said processing unit comprising computerized means adapted to calculate registration data to register the two-dimensional reference systems (G Rif , G′ Rif ), used to express the coordinates of the points of said first image and of said second image, with the three-dimensional reference system (B Rif ), defined by the marker elements of said reference target.