A method of determining a rigid transformation for matching the position of an object with the position of a target object represented by a three dimensional (3D) triangulated wire mesh target model. The method may be used to monitor the positioning of patients undergoing radio therapy or CT (computer tomography) scanning. A stereoscopic image of the object (such as the patient) is processed to identify the 3D positions of a plurality of points on its surface and triangles in the target model surface closest to the identified 3D points are identified. A rigid transformation is calculated that minimises point to plane distances between a selected set of the identified3D points and the planes containing triangles of the target model surface identified as closest to those points. The selected set of points used to determine the rigid transformation is selected on the basis of the determined distances between the identified 3D positions and vertices of the target model identified as being closest to said positions.
