speed v s and of course in so far as the method is activated the method is deactivated for example by withdrawal of the ignition key a detection of speed v greater than the threshold speed confirms a transition 106 which reactivates the step 100 thus returning the system into standby state a detection of speed v less than or equal to the threshold speed confirms the transition 107 which activates a step 108 at each execution of the step 108 a new real image i 1r is captured by means of the camera 10 a new virtual image i 2v of the ground in top view is produced from the real image i 1r as explained above with regard to step 102 starting from the step 108 represented in fig3 the obtaining of the virtual image i 2v confirms a transition 109 which activates a step 110 the step 110 consists in determining points of interest which characterize the new virtual image i 2v as explained above with regard to step 104 the new virtual image i 2v is compared with a former virtual image i 2v the former virtual image i 2v is the last virtual image i 2v which was integrated in the virtual image i 3v on the execution of step 105 or on a preceding execution of a step 115 explained below the new virtual image i 2v is considered equivalent to the former virtual image i 2v in so far as the distance between each point of interest of the former virtual image i 2v and the point of interest which corresponds thereto in the new virtual image i 2v is lower than a predetermined threshold value the distance can concern a euclidian distance on the situation of.