A method of configuring data processing apparatus for automatically determining the real size of an object in an image of a scene is provided. The method includes responding to user input by displaying in the image of the scene displayed on the display screen a virtual object representative of a real size object to establish a first datum position, and storing data defining the first datum position; varying a size of the virtual object in the image; storing data representative of a horizontal and vertical size of the virtual object relative to the displayed image at the first datum position; displaying said virtual object at a second datum position in said image; varying the size of the virtual object on the display screen at the second datum position; and storing data representative of the vertical and horizontal size of the virtual object at the second datum position.