The invention relates to a data processing method of determining a transformation for transforming medical image data into a positional reference system, the method being executed by a computer and comprising the following steps: a) acquiring, from a medical imaging apparatus (5), medical image data comprising medical image information describing a two-dimensional image of an anatomical body part (1); b) acquiring medical image selection data comprising medical image selection information describing a selection (4) from the medical image information; c) acquiring imaging apparatus characteristic data comprising imaging apparatus characteristic information describing an imaging characteristic of the medical imaging apparatus (5); d) determining, based on the medical image data, medical image selection data and imaging apparatus characteristic data, selection position data comprising selection position information describing a three-dimensional position of an anatomical structure (2) in the anatomical body part (1) corresponding to the selection from the medical image information.