Abstract: A method of determining the trajectory of a body (1) such as a vehicle body on a relatively plane path portion (2), the method being characterized in that it consists: in forming a main real image (4) of the path portion in a plane (19) at a non-zero angle (5) with the path portion; in decomposing said main image as formed into a plurality of points (21); in determining the relationship between the size of a unit length (30) taken substantially at the level of the path portion and the size of its image formed in the main image the size being a function of the number of points covered by the image and of the location of the unit length on said path portion; in determining a secondary image (32) in the main image, the secondary image corresponding to a longitudinal reference mark (31) related to the vehicle on the path portion; and in determining the various successive positions (32, 33, 34) of the secondary image by correlation of the number of points covered by the secondary image, given that the secondary im