If you know the positions of four or more points on a flat surface
you can transform the image so as to obtain a perfect plan view of the
surface. Measurements can then be taken from this image.
Image with 5m calibration square.
The positions of the four
corners are used to compute the transformation required.
Note that objects that are not in the plane of
the road surface, such as the dunes appearing at the top, will be distorted.
Measurements can only be applied to the road surface.