A digital video camera is placed on a vehicle. The video camera is attached to the vehicle and is directed to a surface at a certain angle to the surface. The camera angle can change while vehicle is in motion. The camera angle is constantly measured by angular sensor (i.e., inclinometer). Images of the surface are projected onto a video matrix and transformed into a sequence of digital frames that are formed at a pre-set time interval. A speed of the video camera relative to the surface is calculated as a ratio of an off-set between the adjacent frames and the time interval between the frames. Then a known relative speed of the vehicle can be corrected by multiplication of the height of the camera (above the surface) by a coefficient that equals a ratio of the known vehicle speed and the measured camera speed.