1. An input system for selecting a point by a user in a 3D volumetric visualization image of a three-dimensional data set, comprising: a volumetric 3D monitor which shows the 3D volumetric visualization image of said three-dimensional data set in 3D space surrounded by an associated surface or surfaces outwardly spaced from said 3D volumetric 3D space visualization image on which a reference point is definable with respect to said 3D visualization image shown by the monitor; a selection unit to select said reference point on the surface or surfaces relative to the 3D volumetric visualization image on the volumetric 3D monitor of the three-dimensional data set selected by the user, a direction unit to specify a direction from said reference point to said point being selected by the user in the 3D volumetric visualization image on the volumetric 3D monitor, and a distance unit to set a distance value from said reference point along said direction to said point being selected in the visualization image.