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, medical image data comprising focussed medical image information describing a two-dimensional image of an anatomical body part of a patient; b) acquiring medical image selection data comprising medical image selection information describing a selection from the medical image information, wherein the selection from the medical image information is represented by two-dimensional image information; c) acquiring imaging apparatus characteristic data comprising imaging apparatus characteristic information describing an imaging characteristic of the medical imaging apparatus, wherein the medical imaging apparatus characteristic information comprises optical characteristic information describing the focal length of the medical imaging apparatus, characterised by d) acquiring, by detecting at least one marker attached to the patient's body in a predetermined spatial relationship relative to the anatomical body part, anatomical body part position data comprising anatomical body part information describing a position of the anatomical body part in a navigation coordinate system; e) acquiring medical imaging apparatus position data generated by detecting, by a navigation system, at least one marker attached to the medical imaging apparatus with a predetermined spatial relationship relative to the medical imaging apparatus, the medical imaging apparatus position data comprising medical imaging apparatus position information describing a position of the medical imaging apparatus in the navigation coordinate system and wherein the medical image selection data is determined based on the medical imaging apparatus position data; f) determining, based on the medical image data, medical image selection data, anatomical body part position data, medical imaging apparatus position data and optical characteristic information, selection position data comprising selection position information describing a tree-mensona position of an anatomical structure in the anatomical body part, the anatomical structure corresponding to the selection from the medical image information, wherein the selection from the medical image information is mapped onto a three-dimensional model of the anatomical body part, wherein the selection position data is determined based on focal transformation data comprising focal transformation information describing a transformation from the imaging plane of the medical imaging apparatus to the focal point of the medical imaging apparatus; and g) providing a visual display of the selection from the medical image information in a model of the anatomical body part. a) acquiring, from a medical imaging apparatus, medical image data comprising focussed medical image information describing a two-dimensional image of an anatomical body part of a patient; b) acquiring medical image selection data comprising medical image selection information describing a selection from the medical image information, wherein the selection from the medical image information is represented by two-dimensional image information; c) acquiring imaging apparatus characteristic data comprising imaging apparatus characteristic information describing an imaging characteristic of the medical imaging apparatus, wherein the medical imaging apparatus characteristic information comprises optical characteristic information describing the focal length of the medical imaging apparatus, d) acquiring, by detecting at least one marker attached to the patient's body in a predetermined spatial relationship relative to the anatomical body part, anatomical body part position data comprising anatomical body part information describing a position of the anatomical body part in a navigation coordinate system; e) acquiring medical imaging apparatus position data generated by detecting, by a navigation system, at least one marker attached to the medical imaging apparatus with a predetermined spatial relationship relative to the medical imaging apparatus, the medical imaging apparatus position data comprising medical imaging apparatus position information describing a position of the medical imaging apparatus in the navigation coordinate system and wherein the medical image selection data is determined based on the medical imaging apparatus position data; f) determining, based on the medical image data, medical image selection data, anatomical body part position data, medical imaging apparatus position data and optical characteristic information, selection position data comprising selection position information describing a tree-mensona position of an anatomical structure in the anatomical body part, the anatomical structure corresponding to the selection from the medical image information, wherein the selection from the medical image information is mapped onto a three-dimensional model of the anatomical body part, wherein the selection position data is determined based on focal transformation data comprising focal transformation information describing a transformation from the imaging plane of the medical imaging apparatus to the focal point of the medical imaging apparatus; and g) providing a visual display of the selection from the medical image information in a model of the anatomical body part.