1. A method of determining misalignment between a first image and a second image, the first and second images being viewable stereoscopically, the method comprising: determining a feature position within the first image and a corresponding feature position within the second image; defining, within the first image and the second image, the optical axis of the cameras capturing said respective images; calculating the misalignment between the feature position within the first image and the corresponding feature position within the second image using a model, the misalignment being determined in dependence upon the location of the feature position of the first image and the corresponding feature position of the second image relative to the defined optical axis of the respective images; and testing the validity of the calculated misalignment using a random sample consensus technique, whereby the tested misalignment is valid when the random sample consensus technique achieves a predetermined condition, wherein the model is defined by the equation 
 y=S ·( xL ·sin θ+ yL ·cos θ)+ T where y is a vertical position of the feature point in the second image; xL and yL are respective horizontal and vertical positions of the feature point in the first image; θ is a roll angle; S is a scale metric and T is a vertical mismatch metric between the two images.